cenozoic magmatism in kalimatan and its related geodynamic evolution

8/3/2019 Cenozoic Magmatism in Kalimatan and Its Related Geodynamic Evolution

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PERGAMON Journal of Asian Earth Sciences 17 (1999) 2545

Ceno zoic magmatism in Kalimantan and

its related geodynamic evolution

R. Soeria-Atmadja *, D.Noeradi, B. Priadi

Teknik Geologi, Institut Teknologi Bandung, Jalan Ganesa 10, Bandung, 40132, Indonesia

Received 27 November 1997;accepted 29 April 1998

Abstract

The NESW Tertiary magmatic belt of central Kalimantan is related to two

separate periods of subduction; during the Eocen eOligocene and Late

Oligocen eMiocene. The younger magmatic belt is superimposed upon the

earlier belt. This magmatic belt is characterized chie y by Late

Oligocen eMiocene volcanic products, among which limited exposures of the

Eocene volcanics have also been mapped by previous investigators. This calc-

alkaline magmatic belt has become known as the

gold belt' of Central West Kalimantan on account of a number of discoveries of

Neogene epithermal gold mineralization. This mineralization is found in central to

proximal volcanic settings and occurred at relatively shallow depths. The earliest

known subduction-related magmatism took place in the Eocen eEarly

Oligocene with the emplacement of calc-alkaline silicic pyroclastics, followed

by a period of continental collision. Subsequent subduction-related

magmatism continued from Late Oligocen ePleistocene, during which time the

magma evolved from calc-alkaline to potassic calc-alkaline. Plio-Pleistocene

magmatism resulted in the formation of basalt ows. The present available KAr

ages of the Cenozoic volcanics range from 51

to 1 Ma. # 1999 Elsevier Science Ltd.All rights reserved.

1. Introduc tion

The search for gold in central

Kaliman tan during the early

eigh ties has produced valuab le

info rmation concerning the

tecto nic setting and the geological

environ ments of format ion of

the mine ral deposits. A num ber

of epithermal gold occurrences in

the region are dis tributed along a

SWNE trend ing belt within the

Terti ary mag matic arc (van

Leeuwen et al., 1990). This gold

belt repre sents an epithermal

environ ment, whe re mine ralizat ion

occurred most commo nly in cen-

tral to proxi mal volcanic settingsat shal low depth. The host rocks

are typi cally eusives or


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pyrocl astics of inte rmediate to

acidic calc-alka line rock series,

and show feat ures suggesting

derivation from near-n eutral pH

geothe rmal uids. The character

of the ore ui ds and the types of

mine ralizat ion, as well as assoc iated

alt eration indicate thatmineral ization occ urred in a

* Correspon ding author. Tel.: +62-22-250-0970; fax: +62-22-250-

2201; e-mail: rubini@ gc.itb.ac.id.

low sulp hide environ ment. Most

commo nly mine raliz- ation in the

region occ urs in hydrothe rmal

brecc ias, vein stru ctures and quartz

stock works. These volcan ic- hosted

gold deposits are, acc ording to

White and Hede nquist (1990 ),

typical ly epitherma l.

Rock chemistry as well as KAr

ages of the volcan ic host rocks

indic ate that the calc-alka line

mag matism was related to late

Oligoce neEa rly Mioce ne SSE -dip-

ping subduction from the osh ore

region to the north of Kaliman tan

(Ha milton, 1979; Ca rlile and

Mitc hell,1994). Carlile and Mitchell

(1994) identi ed the Te rtiary

mag matic arc by remnan ts of

Ea rly Tert iary andesitic volcanic

centr es with wh ich the

epithermal gold mineral izati on is

assoc iated. Accor ding to their

observations epith ermal gold

mine ralizat ion in

Indonesia is best developed above

contin ental crus t, such as in the

we stern Sunda-Ba nda and the

central Kal imantan arcs,

whe reas porphyry-t ype

mine ral deposits are

for med in island arc as well as in

continental setting s; e.g. the

coppe rgold por phyry of Ba tu

Hi jau, Sumba wa (Me ldrum etal., 1994) and the

1367-9120 /99 - see front matter # 1999 ElsevierScience Ltd. All rights reserved. PII: S 0 7 4 3 - 9 5 4 7( 9 8 ) 0 0 0 6 2 -2
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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2 R. Soeria-Atmadja et al. / Journal of Asian Earth Sciences 17 (1999) 2545

molybde num por phyry of the

Mala la dis trict, northe rn arm of

Sula wesi (van Leeuwen, 1994). Th is

pap er is an att empt to summarize

the avai lable geological data,

contributed by sever al work ers as

the result of the inten sive search

for gold in Cen tral Kaliman tan,

high- lighting some of the feat ures

of the related mag matism and its

tecto nic set ting.

2. Geology of the Tertiary magmatic belt

2.1. Regional geologic setting

Van Be mmelen (1949) noted

the charac teristic fra mework of

arcua te structures of Kaliman tan

island with their convex sides

towards the south and south-

west. These stru ctures are mos tly

EW trend ing in central

Kaliman tan and are bound ed in

the south by the Schwaner

Mountain magmat ic arc, wh ich

is paired with a Cretaceous

mel ange comple x. The region al

geologic set ting of the northe rn

part of Kalima ntan, es- pecia lly

Borneo (Ma laysi an and Bru nei

ter ritories) has been described by

Hutch ison (1996 a) based on pre-

vious work s. He distingui shed

three major tecto nic zones fromthe northwest to southea st, the

Miri, Sib u- Ra jang and Kuc hing

zones (Fig. 1).

Acco rding to Hutch ison (1996 a)

the Miri Zone rep- resen ts part of

the Lucon ia continental block

wh ich is limi ted to the northwest

by the Pala wan Trench, the trace

of a Miocene sub duction zone

(Ha milton, 1979), whe reas thesouthea stern limit has been

referred to as the Mersing Line.

The oldest rock expos ures in this

zone include roc ks of the Long

Bawan and Kal alan for mations

which consi st of stron gly folded

uvio-d el- taic sedi ments of Late

Cretaceou sEocene age. The

overl ying Eo-O ligocene

sedime nts of the

Mulu Fo rmation consi st of

limes tones which, accord ing to

Hutch ison (1996 a), we re

deposited on a relativ ely stab le

contin ental shelf. You nger

Miocen ePliocene sedi ments were

deposited in a delt aic to braided

str eam environ ment in syncl inal

basins formed during a previous

period of fold ing.

The Sibu-Rajang Zone consi sts

of inten sely folded, weak ly

metamorp hosed ysch sedi ments of

the Rajang Group. The Rajang

Group is composed of the Late

Cretaceou sOlig ocene Balaga,

Lurah and Crocker formati ons. The stron gly deformed Upper

Cr etaceou s Eoc ene ysch

seq uence of the Embaluh Group

for ms the south wards exten sion of

the Ra jang Group into

Kaliman tan. These ysch

sedi ments have been inte r- preted

by Hutchis on (1996 a) as

turbi dites wh ich were scraped

into an accre tionary prismduring Late Oligoce ne time, as

the result of convergence of the

Lucon ia continental block with the

northe rn margin of Sund aland.

Neoge ne v olcanics, of basaltic to

daci tic comp osition, overlie the

Pale ogene sedime nts unco n-


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R. Soeria-Atmadja et al. / Journal of Asian Earth Sciences 17 (1999) 2545 2

for mably. The Lupar Sut ure

Line, along wh ich Cretaceous

ophiol ites are exp osed, marks the

bound- ary bet ween the Sib u-

Rajang and the Kuching zones.

Ophio lite mel anges also occur along

the Bukit Mersi ng Li ne and in the

Semitau Ridge. The oph iolites

rep- resent the basem ent of the

Rajang Gro up and indic ate the

trace of a Late

Cretaceou sOligocene subduction

zone. The eastwa rd continuation

of this suture line is not clea r,

possibly it may be con nected

with the no rth south trend ing

Long Ara nWititi thrust fault or

the Adio sutur e.

Acco rding to Hutch ison (1996a)

the Kuching Zone marks the

northe rn margin of Sund aland.

Along this zone Pale ozoic

crystal line schi sts are overl ain by

car- bon ates and siliciclas tic

sedi ments of Pale ogene to

Mesozoic age. The PaleogenePiyab ung Vo lcanics and Neog ene

Sintang Intru sives (Her yanto et al.,

199 3) are the mag matic rocks in

this zone. The southern limit of the

Kuching Zone is marked by an

EW trend ing faul t, which

acc ording to Tanean et al.

(1996) continues as far as the

Ada ng Fault separa ting the Kutai

Ba sin in the north from the Bari to

Ba sin to the south. Th is zone can

possib ly be fol lowed fur ther

eastwa rds into the Up per Kutai

Basin, whe re Neogene and

Pale ogene volcan ics have also

been document ed (Ke lian, Muara

Wahau) and then swings

north wards into the Upper

Tar akan Basin.

2.2. Tertiary volcanic rock association

Carlile and Mitc hell (1994)

tent atively projec ted the Te rtiary

mag matic arc from northea st

Kal imantan southwards through

central and west Kal imant an to

Sarawak, followi ng the southern

bound ary of the Kuc hing High.

Va luable geological informa tion on

the volcanic rock-associa tion,

geolog ic setting and rela ted

mine ralizat ion have been

document ed from sever al districts

along the magm atic arc (among

others Busang, Kel ian, Muyup, Mt.

Muro, Mas uparia, Muara Wa hau

and Sintan g; Fig. 2). The volcan ic

rock series are products of

Early Tertiary calc-alka line

mag matism. Th ree mag matic

episodes have been ide ntied by

previous workers, Eoc ene acid ic

volcan ism was fol lowed by Late

Oligocen eMiocene

andesiti c rhyolit ic volcanism

(prec eding epithermal

mine ralizat ion) and then by Plio -Plei stocene basalt volcan ism; the

latter gave rise to basal tic lava

ows and dykes of region al exten t.

Felderh of et al. (1996) ident ieda maar diatrem e

do me complex and assoc iated

epith ermal gold mineral- iza tion in

the Busa ng area northea st of

Keli an. They described the local

geolog ic setting as a NWSE trend-

ing semi-circ ular gra ben stru cture

of Early Tert iary age, intru ded by

a Mid-Te rtiary maar diat reme

and dacitic do me comple x; the

dome hosts gold mine raliz- ation

along northwest trendi ng frac ture

zones in whi ch the mine ralizat ion

style is due to hydrofr acturing.

The


5/33

Fig.1.Tectonicframework

ofKalimantanandsurroundingislands

(modiedfrom

Taneanet

al.,1996

).


6/33

Fig. 2. Distribution of volcanic outcrops and KAr dated volcanic rocks in Kalimantan.

Data from Kirk, 1968; van de Weerd et al., 1987; Pieters et al., 1987; Tate, 1991; Bellon

and Rangin, 1991; Harahap, 1993; Heryanto et al., 1993; Thomps on et al., 1994; van

Leeuwen, 1994; Carlile and Mitchell, 1994; Tanean et al., 1996. 1, Non-del ineated outcrops;

2, delineated outcro ps; 3, Miocene and Plio-Pleisto cene; 4, Miocene;

5, Oligo- Miocene; 6, Eo-Oligo cene and Miocene; 7, Eocene; 8,KAr ages in Ma; 9, folded belt.

volcan ic membe rs of the maar

deposi ts are acid ic tus, wh ich

include rhyolit ic crystal tu,

andesitic lapilli ash tu and

rhyolit icdacitic pyrocl astic roc ks;

the assoc i- ated sedi ments of

ysch facies consist of met amor-

pho sed muds tones and

clay stones. Acco rding to

Felderh of et al. (1996) the dome

com plex is ma de up larg ely of early

phase volcan ics showing a

grada tional cha nge towards the

periphe ry from mediu m- to coarse-


7/33

grain ed quartz- hornblende dacite

to porphyritic rhyo- daci te, and

are charac terized by a very ne-

grained grou ndmass; the late

phase volcan ics are represented

by ne-gr ained aphyric

andesi tedacite plugs, as well as

subparallel sheet ed dykes of

porphyritic andesi te and dacite.

The se v olcanic rocks are the

products of a phase ofMid-Te rtiary calc-

alkaline mag matism. Fel derhof et

al. (1996) also document post -

mine raliz- ation mag mati sm,

represented by basalt and quart z

rhy olite plugs and dy kes.

In the Kel ian mini ng distric t,

southwest of the Busang area, the

oldest exposed volcanic rocks

occur as a Late Eoce ne silic icpyr oclastic unit, 300 m thick,

includ ing pyrocl astic, as well as

epiclast ic-rocks of


8/33

rhyol itic c omposition (epi clastic,

air-fall as we ll as wel ded

pyrocl astic ow deposits)

contai ning pumice, crys tal and

lithic fragment s.

Acco rding to van Leeuwen et

al. (1990) this unit is overl ain by a

thick pile of Eoce neOlig ocene

sedi ments with local tu ac- eous

intercala tions, cut by rhyol itic

dy kes and sma ll andesitic stocks

and dykes. These int rusions were

empl aced in the Early Mioce ne

alo ng NS and NE SW trendi ng

fractures and faul ted zones andwe re followed by four -stage

sequence of hydrothe rmal activi ty

and mineral izatio n. Intru sions of

inte rmedi ate po r- ph yry cutting

andesitic volcan ic cou ntry rock

at Muyup, close top Kel ian, have

also been report ed by van Leeuwen

et al. (1990).

In the Gu nung Mas area,

south west of Mt. Muro, goldmine ralizati on occurs as gold-

bearing ssure veins and

stockw orks in the contact

zone between a Cr etaceous

granit oid and a djacent

sedimentary roc ks (van Lee uwen,

1994). Gold mine ralizat ion at

Mirah, south west of Gunu ng

Mas, occurs along zones of

quartz stock works, hydrothe rmal

bre ccias and veins foll owing NNW

trend ing frac tures. The host

roc ks consist of stron gly altered

(argil lic) pyrocl astics, bound ed to

the west by an andesi te plug

(van Leeuwen, 1994), possibly of

Eoc ene age.

Van de Weerd et al. (1987)

obtain ed a KAr age range of

24.0 Ma14.4 Ma from nine

samples of andesite and basalt

coll ected from an area between

Kelian and Mt. Muro, whe reas

van Leeuwen et al. (1990) record

an age of 22.9 2 0.5 Ma from a

rela tively fresh

andesite. These volcan ic roc ksreprese nt a phase of lateOligoce neEarly Miocene calc -

alka line mag matism. Post-

mine ralizati on mag matism

resu lted in the extru- sion of

widespread plateau basalts.

Volca nic rock outcrops forming

the remna nts of a volcan ic crat er

at Masuparia, south west of

Kel ian, consist of calc-alka line

intrusive bodies (Diorite, mon-

zonite, gra nodiorite) and many

dior ite plugs, lava

ows, proxi mal pyr oclastic ow

breccias and tus of

andesiti cdacitic composition

(Thom pson et al., 1994). Most of

these volcan ic ows and tus are

plag ioclas e- phyric, with minor

pyr oxene and hornblende.Towards the periphe ry of this

volcanic cente r, the volcanic sedi-

ments are inte rbedded with

sedi ments of the Barito Basin.

Accor ding to Thom pson et al.

(1994) this area repre sents a

stratovo lcan o. Tho mpson et al.

(1994) postu late a three-s tage

magmat ichydrothe rmal system,

in which the second- stage ui ds

we re resp onsible for the

mine ralizat ion. Miner alization is

assoc iated with either NW

trend ing stru ctural zones or a

N9 08100 E8 trend ing line ament. A 24.620.4 Ma KAr agewas obtain ed from these volcan ics.

The wholeasse m- blage is overl ain

by oliv inebasalt ows representing

a Plio -Pleistocene magmat ic event.

A simil ar volcanic rock


9/33

assoc iation is exposed in the

Mt. Muro area, direc tly east ofMasupar ia, where

there is an interbe dded sequence of

subaerial calc-alkaline andesi te

and basal tic andesite ows,

breccias and tu s (Si mmons and

Browne, 1990). These rocks are

porphyritic, consisting

pre dominan tly of plag ioclasephenocrys ts with minor pyroxene

and amphibo le. Sim mons and

Br owne (1990) noted that

conjuga te pairs of NW- and NE-

tre nding shears and faults gen-

eral ly lack mine ralizat ion; ins tead

mine ralizat ion took place along

NNW -trend ing tens ional fracture s.

In co nnection with a region al

study project of the Kutai Ba sin

und ertaken by VICO (Virginia

Indonesia Company), Tan ean et al.

(1996) samp led Tertiary vol-

canics of the Muara Wahau area.

The roc ks of the Muara Wahau

Forma tion are ma de up chie y of

clay with inter calations of

sands tone and volcan ic ma-

ter ials. The lat ter include

hornblend eandesite ows and

dacitic pyrocl astic tus with

andesite dykes. The ir stron gly

por phyritic text ures and rock

chemistry suggest an island-a rc

calc-alka line rock seri es. Tanean et

al. (1996) obtain ed a KAr age

range of 21.2 20.39Ma 16.9 2 0.3 Ma from veanaly sed samp les. Resu lt

of stud ies on the volcan icfrag ments c ontained in the Ea rly

Miocene (N4N7) and Mid dle

Mioce ne (N8 A N10) sandst ones

sugges ts two di erent

source sthe Sintang and Muara

Wa hau volcan ics.

The earliest Tertiary mag mati sm,

represe nted by the Piya bung

volcan ics, is recorded from the

Sintang area (Sem itau Rid ge),for ming the westward

prolonga tion of the mag matic


10/33

arc. Acco rding to Hery anto et

al. (1993) these volcan ics are the

products of subaer ial air -fall and

ash- ow volcanism, composed

chie y of crys tal-lithic tus,

pumice-bearing crystal -lithic tus

and agglo merates, partly welded.These tus have been dated at

49.9 2 1.0 Ma, and are perhaps

correlatab le

with the silicic pyr oclastic unitof Kel ian dis trictrep orted by van Leeuwen (1994).

The distribu tion of Late Eoce ne

volcan ics is sporad ic. Besides

Kel ian, Mirah and the Semitau

Rid ge (Sin tang), Late Eoc ene

volcanics (ac id to inte rmediatetu , agglomerate, igni mbrite and

dacite) are also known from

Ma hakam area of the West Kutai

Basin (Ny aan Volca nics) which

have given K/Ar ages of 48.6 Ma

(Tate, 1991) and

50 22.5 Ma (Pie ters et al., 1987).Eoc ene volcan ics

near Singka wang (dacites of the

Serant ak Volcanic s) gave a K/Arage of 51.3 Ma (Hutchison,

1996b) and those of the Mandai

Ba sin from the Muller Mountains

were dated at 40.9 Ma (Pieters et

al., 1987).

Pro ducts of subseque nt

mag matism in central west

Kal imantan occur as iso lated

outcrops of hig h-level stock s, sills,

dykes and pl ugs, referred to as

the Sintang Intru sives of the

Ketun ggau Basin and the east ern

part of the Mela wi Basin

(Wi lliams and Harah ap, 1987;

Harah ap, 1993; Hery anto et al.,

1993). Petro graphic types vary

from granitoid rocks, dacite,

dacite porphyry, andesite, with

minor rhyol ite, rhy oda- cite,

dolerite, basalt and gabbro. Based

on their geo-


11/33

logi cal setting and the relative

abundances of volcanic rock types,

Harah ap (1993) dis tinguis hed

three dier- ent province s. From

the chem ical analy ses (Hara hap,

1993), the southe rn p rovince

(Ear ly Cretaceous base- ment)

shows pred ominantly basalt and

rhyol ite, the centr al province

(Me lawi Basin) is charac terized

mos tly by andesi tes and daci tes

(without basalts ), whe reas the

northe rn province (Ke tunggau

Basin) has abund ant basal tic

andesites but no rhyolites. Twe lveK/Ar dates on separated mine rals

gave an age range of 30.4 Ma

16.4 Ma, corre spondi ng toOligoce neMiddle Miocenemag matism (Wi lliams and

Harah ap, 1987); the age range

ass igned to the Muara Wahau

volcan ics of Upper Kutai district

also fal ls within the same age

range (Tanean et al., 1996). Eight

rock samples were coll ected from

the Tertiary Sintang Intru sives and

ana- lysed by Pro uteau et al.

(1996). They include six grano-

dior ites of adakitic type, yiel ding

two K/ Ar ages 18.3

Ma and 19.2 Ma, and t wo calc-

alka line dacites of younger age

(16.5 Ma and 16.7 Ma).

Accor ding to Prouteau et al.

(1996) the adakites are related to

the incip ient subduction of the

Proto-Sou th Ch ina Sea,

whe reas the lat er calc-alka line

dacit es represe nt a more advanced

stage of the subduct ion proces s.

The inte rior of Bor neo, along the

border of Sara wak andKal imanta n, is occu pied by

platea ux of Cenozo ic lavas and

pyrocl astic cones. Acco rding to

Hutch ison (1996 a), this volcan ic

arc should be paired with the

Nor thwe st Borneo trench,

repre senting a NW- facing

subduction system. The bulk of

this volcan ic ser ies appears to be

of Miocene age. These platea ux

rise to altitu des of up to 2600 m

and consi st main ly of calc-al-

kaline to potassic calc-alka line

volcani cs, includi ng rhy odacites,

andesi tes and basal ts. Dac itic,

igni mbrite- tu s, glassy lavas and

brecc ias are impo rtant products of

this phase of volcan ism. Ki rk

(1968) reports K/Ar ages vary ing

from 25 Ma to 4 Ma. This

volcan ic arc can be trac ed furth er

NE to the Dent Peninsula of

Sabah, where the volcan ics

occur within Late

Miocene-E arly Plioce nerocks

of the Tungku

Fo rmation (Ha ile et al., 1965).

A sum mary of Tertiarymagm atism in Kal imant an is given

in Table 1.

Table 1

Summary of Cenozoic magmatism in Kalimantan

Period Magmatism

and related

tectonic

environm ent

Distribu tion of volcanic products

Plio-Pleis tocene Within-pla temagmatism

(tholeiitic)


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Late Miocen ePleistoc ene Subduction-r elated

(calc-alkali ne somehigh Mg)

Middle Miocen ePliocene Subduction-r elated

(high Kcalc-alkali ne)

Late Oligocen eMiddle MioceneSubduction-r elated

magmatism

(calc-alkali ne)

Eocen eEarly Oligocene Subduction-r elatedmagmatism

(calc-alkali ne)

Busang: basalt plugs and dykes;

Kelian and Mt. Muro:

plateau basalts;

Masup aria: olivine-b asalt

ows; Sintang: basalts;

Usun Apau:

basalt ows;

Linau-B alui:

basalts.

Dent and Semporna

Peninsula s: ows, tus

and basalts;

Upper Tarakan

basin: andesites,

basalts and tus;

Sulu Ridge: a sequence of

andesites, basaltic ows

and pyroclastic rocks.

Usun Apau: dacite andignimbrite tus;

Nieuwe nhuis Mt:

andesites and basalts;

Kinabal u: granito ids

intrusions;

Hose Mt.: dacite

tus and lavas;

Linau-B alui Plateau;

dacites, andesites and basalts;

Cagayan ridge: basalts and

porphyri tic andesites. Busang:

acidic tus (andesites, dacite s,rhyolites ); Masup aria: granitoid

intrusions,

andesite- dacitic pyroclastic and ows;

Mt. Muro: andesite and basaltic andesite

ows breccia and tus; Kelian and

Muyu p: andesite stocks, dykes;

Muara Wahau: andesi te ows and

pyroclastic tus (dacite), dacitic

dykes;

Sintang: basalts, andesites,

dacites, rhyolites.

Singkawa ng: dacites;

Manda i: dacites;

Piyabung: pyroclastic

tus and breccia; Kelian:

silicic pyroclastic

(rhyolit e);

Nyaan: agglomera tes, ignimbrites anddacites.


13/33

Fig. 3. Diagram showing major elements versus SiO 2 from the volcanic rocks

of Kalim antan.


14/33

Fig. 4. Rock anity after Peccerillo and

Taylor (1976). Symbols asin Fig. 3.

3. Chemical characteristics of the volcanic rock

For the purpose of this paper

availa ble analytical data on the

rock chemistry of the Tert iary

volcan ics (Table 2 (a)(d)) from

various districts in Kaliman tan has

been used, among other s, the

Oli go-Mio cene volcanics of

Masupar ia (Thompson et al.,1994); Mou nt Muro (Simmo ns and

Browne, 1990); the Sintang area,

Cen tral Kal imant an (He ryanto et

al., 1993) and the Miocene to Plio -

Pleistoce ne v olcanics of Sabah

(Bellon and Rang in, 1991). Major,

trace a nd rare earth el- ement

data are available on ly for the

Oligoce ne Miocene volcan ics,

whe reas for the Miocene toPliocene -Plei stocene volcan ics only

major eleme nt analyses are

avai lable. Eoc eneEarly Oligoce ne

and Plio- Plei stocene mag matism

are not supp orted by rock

chem istry (Ta ble 1). Analysis with

LOI values (loss on ignition) higher

than 4 have been excluded, to

av oid the eects of secondary

alterat ion. In general, chem ical

patt erns disp lay posi tive

corre lations bet ween Al 2O3, K2O,

Na2O, MnO and P2O5 with

incre asing silica content, wh ile

TiO 2, Fe2O3, MgO and CaO

conten ts decrea se (Fig. 3). In the

K2OSi O2 diagr am (Peccer illo andTaylor, 1976) rock anities range

from tholeii tic and calc -alkaline to

high- K. Tho leiites are poorly rep-

resen ted; calc-alka line roc ks

being most common (Fig. 4).

Gener ally the chem ical patterns

of the Tert iary volcanics in the

region are characteriz ed by high

alumina contents, varyi ng between

12. 619.4 %, low conten ts of

MgO, alka lies (Na2O + K2O less

than 8%) and Ti O2

(0. 05 1.28 %), enriched patt erns

of Chrond rite-norm al- ized spider

diagr ams, with negat ive

anomalies in HF SE, some times

acc ompani ed by enrich ment of

Th and U. The above chemical

feat ures are charac teristic ofsub duction-re lated mag matism on

an act ive continental margin,

except that variations in the Zr

and Y


15/33

contents are more charac teristic ofwithin -plate basalts

(Pearce and Norr y, 1979) (Tables2a1, 2a2, 2b, 2c1

2c3,2d).

3.1. OligoceneMiocene

Rel atively fresh volcanics fromMount Muro (50 .0

62.5% Si O2) and Masuparia

(55 .361.3% Si O2) h ave high

pot assium conten ts, a nd have

potas sic calc-alkaline to

shosho nitic a ni ties, wh ich may

indic ate vary- ing degrees ofcrus tal conta mination. The se

potas sic roc ks show enriched

spider diagr ams, 815 times

MORB and 10 times MO RB, in

wh ich compa tible el- ement

patt erns are rather steep. Tho se

from Mount Muro are

charac terized by negative

anomalies in Sr, Nb, Ti, and V,

while those from Masupar ia

show negative anoma lies in Nb,

with relativ ely enriched Sr, Pb,

and U conten ts.

The cogenetic nature of the

volcan ic roc ks in the region is

evident from overla pping patterns

in the spider diagr am, as

illustrated by basal ts, basaltic -

andesites, andesi tes and dacites

(Fig. 5) from the Sintang area.

They show similar patt erns in their

extended spider diagr ams. The ir

modera tely steep patt erns show

di erent (La/Yb)N ratios of

618, 720 and 922, in roc ks

from the centr al, northe rn and

southe rn parts of Sintan g,

respec tively. They sh ow enriched

patterns, with negat ive anoma lies

in Ti, rela ted to FeTi oxide

frac tionatio n, and Nb rela ted to

sub duction proces ses (Wi lson,

1989). High conten ts of Th and U

may indicate a contribution from

contin ental mat erials.

Ho wever, sever al basaltic roc kscollected from the

Sintang area have higher MgO(higher than 6%) and Ti O2

(1.5 62.26 %) contents. A high

MgO content normal ly corre lates

with a pri mary mag ma

comp osition, wh ile a high TiO 2

indic ates possible crustal co ntami-

nation; both charac teristi cs are

compar able with those of

continental basalts (Wi lson, 1989).

These basalts have not been dated;

perhaps they repre sent products of

a younger mag mati sm episode,

compared to the major ity of the

volcanic rocks in the region.

Fig. 5 demons trates that the

rhyolite data from the Sintang

area show a distinct pattern in

that the (La/ Yb)N rati os range

between 150 600, to give steep

patterns cutti ng across the

curves for other rock types types.

Gener ally, they show low

contents of HREE with

fractiona tion eects on the Ti

content. This volcanic suite

shows the c haracteri stics of

subd uction-relat ed mag mati sm.

Several rock sampl es show ada kitic

characters (Defant andDru mmond, 1996) with Y < 10

pp m, norma lized Yb < 5 and

Sr>500 pp m. Their low HREE

contents show the eects of

met asomatism on the adakitic

mag ma prior to melting, as

sugges ted by Prouteau et al.

(1996 ). Adak itic mag matism took

place at 19.2 818.31 Ma,

fol lowed by subd uction-re latedmag matism between 16.7 216.49

Ma (K/Ar age).


16/33

Table 2a1

Chemical analyses of Tertiary volcanics from Kalim antan(a) Masuparia (Thompson et al.,1994), Oligo-Mi ocene

924-25A 924-22A 924-23A 924-26A 924-27A 924-27BOND 1.142 OND1.141 OND1.165 OND1.264

Si O2 55.53 57.97 61.26 53.37 52.50 52.82 53.96 52.29 54.02 50.59TiO 2 0.99 0.55 0.57 0.92 0.93 0.93 0.75 0.83 0.7 0.89Al 2O3 17.58 16.98 16.61 18.57 18.41 18.61 14.70 15.86 15.54 16.27Fe 2O3 9.03 7.00 5.53 10.16 10.40 10.37 6.46 4.88 7.6 8.96

MnO 0.18 0.18 0.13 0.02 0.02 0.02 0.33 0.26 0.2 0.23MgO 4.05 3.92 2.39 3.98 4.02 4.07 4.46 3.04 3.8 5.06CaO 8.24 7.42 5.44 8.85 8.86 9.05 4.62 4.77 3.2 2.72Na 2O 2.60 3.06 3.62 3.13 3.11 3.07 2.00 0.10 2.9 0.70K2O 0.69 1.71 2.39 0.60 0.59 0.57 3.34 5.43 3.1 5.82P2O5 0.13 0.14 0.14 0.14 0.13 0.13 0.13 0.15 0.1 0.13SO 3 0.02 nd 0.01 0.01 0.01 0.01 0.25 2.95 2.4 0.47LOI 1.57 1.20 1.43 0.49 0.48 0.43 8.93 8.35 5.8 7.73

Total 100.61

100.13

99.52 100.24

99.46 100.08

99.93 98.91 99.82 99.57

Rb 12 5 7 9 1 9 108 17 106 183Ba 281 442 477 147 142 146 501 1023 405 623Sr 361 643 479 489 488 484 173 8 238 8La 12 2 2 9 6 7 1 1 1 1Ce 23 3 4 2 1 1 2 4 1 2

PrNd 14 1 2 8 1 1 1 2 1 1Sm

EuGd

DyEr

YbY 31 1 2 2 2 2 3 3 2 2Zr 108 8 156 6 6 6 122 12 128 9Nb 2.4 3.5 5.7 1.8 2.5 1.6 2. 3.1 2. 2Sc 28 1 1 2 2 2 2 2 2 2V 263 175 125 250 246 249 179 22 374 283Cr 128 2 1 7 7 5 1 1 2 2Ni 48 1 6 9 1 9 4 4 8 1

Th 4 8.1 2.4 2.8 0.6 4. 2 4. 2.Pb 6 1 9 5 7 5 1 1 1 4Zn 89 6 4 6 6 6 9 5 8 8Cu 36 7 3 4 4 4 146 3 4 7U 1.4 1 2.9 1.5 1. 2.Ga 17 1

614

20

19

20

15

16

15

15


17/33

Table 2a2

924-25A 924-22A 924-23A 924-26A 924-27A 924-27BOND 1.142 OND1.141 OND1.165 OND1.264

Si O2 55.53 57.97 61.26 53.37 52.50 52.82 53.96 52.29 54.02 50.59TiO 2 0.99 0.55 0.57 0.92 0.93 0.93 0.75 0.83 0.7 0.89Al 2O3 17.58 16.98 16.61 18.57 18.41 18.61 14.70 15.86 15.54 16.27Fe 2O3 9.03 7.00 5.53 10.16 10.40 10.37 6.46 4.88 7.6 8.96MnO 0.18 0.18 0.13 0.02 0.02 0.02 0.33 0.26 0.2 0.23MgO 4.05 3.92 2.39 3.98 4.02 4.07 4.46 3.04 3.8 5.06CaO 8.24 7.42 5.44 8.85 8.86 9.05 4.62 4.77 3.2 2.72Na 2O 2.60 3.06 3.62 3.13 3.11 3.07 2.00 0.10 2.9 0.70K2O 0.69 1.71 2.39 0.60 0.59 0.57 3.34 5.43 3.1 5.82P2O5 0.13 0.14 0.14 0.14 0.13 0.13 0.13 0.15 0.1 0.13SO 3 0.02 nd 0.01 0.01 0.01 0.01 0.25 2.95 2.4 0.47LOI 1.57 1.20 1.43 0.49 0.48 0.43 8.93 8.35 5.8 7.73

Total 100.61

100.13

99.52 100.24

99.46 100.08

99.93 98.91 99.82 99.57

Rb 12 5 7 9 1 9 108 17 106 183Ba 281 442 477 147 142 146 501 1023 405 623Sr 361 643 479 489 488 484 173 8 238 8La 12 2 2 9 6 7 1 1 1 1Ce 23 3 4 2 1 1 2 4 1 2PrNd 14 1 2 8 1 1 1 2 1 1Sm

EuGd

DyEr

YbY 31 1 2 2 2 2 3 3 2 2Zr 108 8 156 6 6 6 122 12 128 9Nb 2.4 3.5 5.7 1.8 2.5 1.6 2. 3.1 2. 2Sc 28 1 1 2 2 2 2 2 2 2V 263 175 125 250 246 249 179 22 374 283Cr 128 2 1 7 7 5 1 1 2 2Ni 48 1 6 9 1 9 4 4 8 1Th 4 8.1 2.4 2.8 0.6 4. 2 4. 2.Pb 6 1 9 5 7 5 1 1 1 4Zn 89 6 4 6 6 6 9 5 8 8Cu 36 7 3 4 4 4 146 3 4 7

U 1.4 1 2.9 1.5 1. 2.Ga 17 16

14

20

19

20

15

16

15

15


18/33

Table 2b

Mount Muro (Simmons and Browne, 1990), Oligo-Mioc ene

45- 64-114 71- 80- 74-168 17- 17-107 74145 47-

Si O2 50.54 55.86 56.45 55.84 56.96 54.21 57.66 61.56 65.0TiO 2 0.93 0.90 0.87 0.84 0.72 1.03 0.95 0.67 0.6Al 2O3 18.62 17.08 17.16 16.57 16.48 16.43 17.73 17.04 16.9Fe 2O3 8.99 7.95 7.49 7.12 8.70 8.55 8.74 3.94 3.7MnO 0.98 0.47 0.28 0.30 0.22 0.51 0.05 0.19 0.1MgO 4.91 3.37 3.12 2.93 2.72 5.24 1.39 3.30 1.0CaO 8.11 5.66 5.96 4.09 3.05 0.53 0.32 0.26 0.0Na 2O 2.94 3.73 3.39 2.84 3.64 2.63 0.47 1.66 0.0K2O 1.29 2.05 2.57 3.73 2.76 3.81 4.46 6.52 5.1

P2O5 0.22 0.22 0.23 0.30 0.18 0.29 0.20 0.18 0.0LOI 2.17 1.75 1.83 3.65 2.78 5.28 7.72 3.61 5.9H2O 0.19 0.19 0.16 0.41 0.16 0.09 0.24 0.27 0.6

Total 99.89 99.23 99.51 98.62 98.37 98.60 99.93 99.20 99.4Rb 3 4 5 8 7 106 136 154 13Ba 336 277 381 966 502 415 236 870 59Sr 671 525 518 761 392 113 5 177 6La 2 1 1 2 1 1 1 1 2CePrNd

Sm

EuGd

Dy

ErYbY 2 2 2 2 2 2 2 2 1Zr 131 156 162 174 174 138 147 168 12Nb

ScV 270 189 198 126 158 208 170 139 17Cr 2 6 9 7 0 6 7 1 1Ni 1 7 8 9 8 0 0 2 1Th 9 0 1 1 6 0 0 0 6Pb 1 1 2 1 2 2 3 2 2Zn 9 102 9 124 162 115 3 168 6Cu 612 3 4 4 239 5 1 6 3UGa


19/33

Table 2c1

Sintang, Central West Kalimantan (Harahap, 1993; Heryanto et al., 1993), Oligo-Mioc ene

69292 69293 69295 69294 69299 69303 69301 69355 69350 69375 69372 6936869365 69366 69379 69307 69343 69341 69347

Southern area Central area

Si O2 46. 48. 48. 49. 51. 51. 55. 61. 63. 69. 71. 72. 75. 75. 75. 60. 60. 61. 62.

TiO 2 2. 0. 0.9 1. 1. 1. 1. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.3Al 2O3 15. 16. 17. 19. 18. 17. 17. 16. 15. 16. 15. 16. 12. 13. 12. 17. 16. 17. 16.Fe 2O3 15. 9. 9.7 10. 9. 7. 7. 5. 3. 2. 1. 1. 1. 0. 1. 5. 4. 7. 3.9MnO 0. 0. 0.1 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.8MgO 5. 8. 7.5 4. 4. 3. 2. 0. 1. 0. 0. 0. 0. 0. 0. 3. 2. 1. 2.8CaO 8. 9. 9.7 10. 7. 6. 6. 4. 4. 3. 0. 1. 0. 0. 0. 6. 3. 1. 5.2Na 2O 3. 2. 2.3 3. 3. 4. 4. 3. 3. 3. 4. 4. 3. 3. 3. 3. 4. 5. 4.1K2O 0. 0. 0.5 0. 1. 1. 1. 2. 1. 1. 2. 2. 4. 4. 3. 0. 1. 0. 0.8P2O5 0. 0. 0.2 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.1LOI 2. 0. 0. 1. 2. 1. 1. 4. 5. 1. 2. 2. 1. 1. 1. 2. 4. 2. 3.2

Total 100. 97. 96.96 99. 95. 99. 99. 99. 99. 99.67 99. 99. 99.83 99.89 100.61Rb 13 8 12 24 58 20 12 40 39 41 82 60 137 120 112 27 59 20 42

Ba 473 95 220 366 467 360 155 430 453 574 758 674 60 766 801 247 460 423 294Sr 508 329 603 818 798 1172 343 624 694 639 413 416 45 72 113 492 768 260 536La 28 9 10 9 21 29 15 25 28 16 22 18 117 43 28 10 30 27 11

Ce 69 23 22 22 53 67 33 53 61 30 46 41 140 78 49 22 68 66 18Pr 8. 3. 7. 4. 2. 2.1Nd 40 14 15 16 28 32 18 23 25 13 16 19 99 29 14 11 28 33 9Sm 9. 3. 5. 3. 2. 1.7Eu 3. 1. 1. 1. 0. 0.6Gd 9. 3. 4. 4. 2. 1.7Dy 8. 4. 3. 4. 2. 1.5Er 5. 2. 1. 2. 1. 0.9

Yb 4. 2. 1. 2. 1 0.7Y 46 25 19 20 21 21 27 18 17 9 13 6 89 28 16 13 15 38 10Zr 269 88 78 78 208 199 100 145 167 93 143 48 549 81 77 89 167 187 0Nb 8 2 3 3 12 12 6 13 10 4 6 8 34 12 11 3 16 8 3Sc 29 36 33 25 18 12 25 10 11 9 2 2 6 1 5 18 13V 316 187 272 243 174 147 186 84 71 49 10 3 3 8 15 119 99 64 80Cr 33 281 26 9 36 3 151 2 22 10 2 2 2 2 2 71 41 13 92

Ni 58 194 35 23 20 7 133 4 15 6 1 1 2 1 1 24 20 9 35


20/33

Y 1 11 19 6 9 6 8 1 7 2 4 19 42 18 4Zr 16 87 223 10 114 82 114 28 51 1 28 12 235 17 265Nb 1 3 9 3 4 2 5 4 6 1 1 5 9 14 1Sc 1 9 7 6 9 9 10 3 2 3 3 21 21 15 1V 9 66 21 50 57 52 62 3 4 327 23 15 185 98 5Cr 6 22 2 29 50 44 35 3 2 258 8 15 40 57 5Ni 4

319 1 28 31 29 26 1 1 177 6 8 34 33 3

Table 2c2

69345 69344 69481 69340 69342 69338 69325 69336 69327 69337 69363 6936169362 69297 69308 69306 69310 69334 69328

Central area Northern area

Si O2 62. 62. 64. 64. 64. 64. 65. 65. 65. 65. 72. 72. 73. 49. 54. 56. 56. 63. 63.TiO 2 0.4 0. 0.3 0. 0. 0.4 0. 0. 0. 0. 0.0 0.0 0. 1.9 1.5 0.8 1. 0. 0.8Al 2O3 17. 15. 16. 16. 17. 16. 16. 15. 15. 16. 15. 15. 15. 15. 15. 18. 16. 15. 15.Fe 2O3 4.0 4. 3.4 3. 4. 2.5 2. 3. 2. 3. 0.9 0.9 0. 10. 10. 6.8 7. 4. 5.6MnO 0.1 0. 0.0 0. 0. 0.0 0. 0. 0. 0. 0.0 0.0 0. 0.1 0.2 0.1 0. 0. 0.1MgO 1.7 2. 2.9 2. 0. 1.9 1. 2. 1. 1. 0.3 0.2 0. 7.6 4.1 4.1 3. 2. 1.1CaO 6.2 5. 4.5 4. 3. 4.1 4. 4. 4. 4. 2.5 2.7 1. 9.7 6.6 7.2 6. 4. 3.4

Na 2O 3.6 3. 4.4 5. 4. 4.6 4. 3. 3. 5. 4.2 4.2 4. 2.7 3.2 3.8 3. 3. 4.9K2O 1.3 3. 1.0 1. 1. 1.7 1. 1. 0. 1. 2.3 2.2 0. 0.1 1.3 1.1 1. 2. 1.8P2O5 0.1 0. 0.1 0. 0. 0.1 0. 0. 0. 0. 0.0 0.0 0. 0.4 0.4 0.2 0. 0. 0.2LOI 1.5 1. 2.0 1. 1. 1.5 2. 2. 4. 0. 0.8 1.0 1. 0.0 2.5 0.2 1. 0. 2.1

Total 99.42 100.76 99.60 99.91 99.96 98.60 99.88

Rb 63 2 29 3 55 28 48 90 32 1 4 39 41 108 73Ba 518 274 344 309 399 358 21 589 520 117 332 293 257 1078 387Sr 706 1060 714 1099 526 838 992 504 538 503 323 556 333 475 298La 28 1 28 1 5 12 18 2 15 8 26 16 19 22 25Ce 59 2 58 2 27 24 36 4 34 21 65 34 43 46 58Pr 6. 3 6. 0.3 3.0 4.1Nd 24 1 25 9. 11. 9 15 1.8 14 16 33 17 25 19 31Sm 4. 2 4. 0.4 3.7 3.Eu 1. 1 1. 0.2 1.5 1.Gd 3. 2 3. 0.4 4.1 3.

Dy 2. 2 2. 0.1 3.8 3.Er 1. 1 1. 0.1 2.0 1.Yb 1. 1 1. 0.1 1.3 1.


21/33

Table 2c3

PR068B BH200A AM076A PR077B DT053B PW108A DS071B PP001A

SS108A DS079A CP206B PW110A CP274A Northern area

Si O2 (%) 64.27 61.91 65.68 65.69 65.75 63.53 65.87 72.80 73.87 54.00 56.4656.76 63.47

TiO 2 0.41 0.75 0.35 0.38 0.32 0.88 0.34 0.03 0.07 1.56 0.85 1.28

0.66

Al 2O3 16.88 17.70 16.47 15.75 15.48 15.80 16.68 15.77 15.52 15.32 18.78 16.75

15.93Fe 2O3 3.77 7.48 2.74 3.45 2.67 5.62 3.13 0.93 0.93 10.84 6.86 7.95

4.96

MnO 0.05 0.16 0.04 0.07 0.05 0.13 0.05 0.04 0.05 0.28 0.11 0.120.09

MgO 2.01 1.13 1.71 2.21 1.57 1.18 1.73 0.25 0.35 4.13 4.13 3.322.57

CaO 4.31 1.65 4.30 4.33 4.47 3.44 4.50 2.72 1.98 6.69 7.27 6.894.68

Na 2O 5.26 5.53 4.47 3.90 3.77 4.97 5.18 4.22 4.86 3.23 3.83 3.36

3.78

K2O 1.08 0.86 1.47 1.40 0.99 1.85 1.61 2.21 0.59 1.31 1.19 1.39

2.48

P2O5 0.19 0.50 0.15 0.13 0.11 0.26 0.19 0.04 0.10 0.47 0.21 0.29

0.29LOI 1.68 2.94 2.50 2.58 4.65 2.16 0.49 1.07 1.49 2.51 0.27 1.30

0.98

Total 99.91 100.61 99.88 99.89 99.83 99.82 99.77 100.08 99.81 100.3499.96 99.41 99.89

Rb (ppm) 22 20 38 55 28 73 48 90 32 4 39 41108

Ba 274 423 309 399 358 387 21 589 520 332 293 2571078

Sr 1060 260 1099 526 838 298 992 540 538 323 556 333475

La 12 27 13 5 12 25 18 2 15 26 16 1922

Ce 26 66 29 27 24 58 36 4 34 65 34 43

46Pr 2.89 0.38 4.19

Nd 12 33 9.4 11.3 9 31 15 1.88 14 33 17 2519

Sm 2.06 0.46 3.7

Eu 0.73 0.22

1.19 Gd 1.89 0.48

3.48 Dy 1.61

0.19 3.36 Er 0.94

0.17 1.81 Yb 0.73

0.17 1.52

Y 11 38 6 9 6 46 8 1 7 47 19 42

18Zr 87 187 107 114 82 265 114 28 51 285 120 235

170

Nb 3 8 3 4 2 11 5 4 6 10 5 9


22/33

14

Sc 9 2 6 9 9 18 10 3 2 30 21 2115

V 66 64 50 57 52 53 62 3 4 235 152 18598

Cr 22 13 29 50 44 5 35 3 2 88 15 4057

Ni 19 9 28 31 29 3 26 1 1 6 8 34

33


23/33

Table 2d

Sabah, Dent and Semporna (Bellon and Rangin, 1991), Miocen ePleistocene

13 S.8792 12 S.129 11 S.8411 10S8734 9S264.1 5 S 266 6 S 266 4S87.91 3S87.901S138.1

Sabah/Kinabalu Dent Semporna

Si O2 (%) 58.40 52.00 60.20 48.50 53.10 62.20 62.20 62.35 64.30 59.5TiO 2 1.06 2.27 0.64 0.69 0.98 0.57 0.64 0.50 0.54 0.7Al 2O3 16.19 14.12 16.65 18.92 17.58 15.85 16.69 15.98 15.53 16.0Fe 2O3 7.56 13.10 5.92 8.06 8.26 5.31 5.44 5.88 5.26 6.6MnO 0.15 0.15 0.14 0.17 0.14 0.10 0.08 0.11 0.10 0.1MgO 3.39 6.07 2.39 5.32 3.75 2.64 1.92 2.46 2.23 2.9CaO 4.47 6.60 5.51 10.03 8.92 5.13 6.51 6.00 4.84 6.0Na 2O 3.48 3.06 2.98 2.92 2.54 3.21 3.01 2.73 3.06 2.8K2O 1.58 0.25 2.85 0.48 1.52 2.30 2.11 2.31 2.53 1.6P2O5 0.25 0.15 0.20 0.05 0.30 0.20 0.25 0.12 0.10 0.1LOI 0.69 0.80 1.69 4.91 2.11 2.44 0.47 1.72 1.74 1.9

Total 97.22 98.57 99.17 100.0 99.20 99.95 99.32 100.1 100.2 98.6Rb 55 9 1 6 6 100 8 8 9 5

Ba 371 7 409 7 335 439 365 312 349 430Sr 350 162 466 326 457 482 423 270 245 210LaCePrNd

Sm

EuGd

DyEr

YbY

Zr

Nb

ScV 80 167 153 280 271 121 167 140 120 182Cr 68 232 1 3 1 3 2 1 1 3Ni 40 150 8 1

619

19

17

8 8 9

3.2. Middle to Late Miocene

Middle to Late Miocene rocks

are known from the Dent and

Samp orna peninsulas. They are

mostly inte r- medi ate in

compos ition (dacites to

andesi tes) with minor basaltic

rocks. The low Ti O2 and MgO and

the high alka li content are

indic ative of potassic calc -alka-

line anity and suggest a generic

rela tionship to subduction.

3.3. Late MiocenePliocene

Late Mioce nePlioce ne mag matic

act ivity gave rise to the for mation

of the mag matic rocks of Sabah

and Kinab alu. The dior ites of

Mount Kinab alu exhibit a normal

to potas sic calc-alka line ani ty,

with the charac teristi cs of

subd uction-re lated mag matism.The Pliocene volcanics consist of

thole iitic basal ts, some of wh ich

disp lay relativ ely high MgO

(high er than 6%) and TiO 2

(high er than 2.0% ); these

major eleme nt


24/33

characteri stics point to

mag matism related to within-

plate geody namics.

Table 1 gives a sum mary ofCenozo ic mag matism

and possib le cha nges in the

respec tive tect onic environ- ment,

as inte rpreted from the main

chem ical signa tures revea led by

geochemistry and rock suite

ani ties.

4. Geodynamic interpretat ion

Based on the present data

concerning Te rtiary magmat ism

and the region al geology of

Kal imantan (inc luding Borne o)

four major periods of geodynam ic

evolution have been disting uished as

fol lows:

Acco rding to Dain es (1985)

rifting of the marg in of As ia to

pr oduce the South Chi na Sea

took place during

Eoc eneOligoce ne time, resu lting in

southeast- wa rd mo vement of the

Lucon ia continental block and

lead ing to the sub duction of part

of the South Chi na Sea Plate

beneath the northe rn marg in of

Sund aland.


25/33

Fig. 5. Chondrite normalized extended spider-di agram of volcanic rocks (normaliza tionvalues after Sun and McDonou gh, 1989).

The Eoc eneEarly Oligoce ne

mag matic belt along this

continental margin can be follow ed

from Sintang (central West

Kaliman tan), through Kel ian to

the Up per Tara kan Basin. This

mag mati sm is conside red to be re-

lat ed to southea sterly

subduct ion in the Rajang

Tre nch. The proposed subduct ion

mo del is comparable to an active

contin ental margin of Chili an type

(Fig. 6).

Collision and doc king of the

Luconia contin ental block with

the northe rn margin of

Sund aland must have taken place

during the Middle Olig ocene, as

the youngest sedi ments involved in

the inten se foldi ng ofthe Rajang

Trench are of Early Oligoce ne

age (Hu tchison, 1996a).Defo rmation at this time is also

recorded by the Early Olig ocene

unco nformity in the Mela wi Basin

(Pieters et al., 1987). The suture

related to this collision, the Lupar

Line, is marked by imb ri- cated

ophi olites (Fig. 7).

Late Oligocen eMiddle Miocene

subduct ion-re lated mag matism is

superimposed on the Eoc ene

mag matic belt, and may be trac ed

from Sintang (the western most

part of the mag matic belt)

eastwa rds through

Masu paria and further

north wards to Mou nt Muro,

Kelia n, Muyup, Busang, Muara

Wahau and along the northe rn

ank of Mangkali hat and

Sesayap, Middle Miocen ePlioce ne

volcan ics are doc umented from

the region of the fossil Ra jang

Tre nch (Mo unt Hose,UsunApau and Niewenhu is), to


26/33

Kinab alu and the Cag ayan Ridge,

while Late Miocene to Pleistocene

mag matism extends from the

Upper Tar akan Basin,

into the peninsulas of Dent

and Samporna and throu gh the

Sulu Arc to Mindanao.

The Late Oligoce neMiddle

Miocene mag matic ac- tivity is

perhaps related to remna nts ofthe Eoce ne subducted slab,

whe reas the Middle

Miocen ePlioce ne mag matic

activi ty along the Sibu-Rajang Zone

and Cag ayan Ridge is rela ted to

subduct ion in the Palaw an Trench

(Fig. 8). Th is Mioce ne subduction

could be the resu lt of

countercl ockwise rotation of

Bo rneo, as proposed by Hall(1996 ). Miocene subd uction-re lated

mag matism is also recorded from

Mo unt Kinab alu, the subma rine

oceanic ridge of Cag ayan and

Pan ay Isl and in the Phil ippines

(Bellon and Rang in, 1991).

In Cen tral Kalima ntan Miocene

volcan ic rocks are repre sented by

the Sintang Intrusives of potas sic

calc- alka line ani ty (He ryanto et

al., 1993) which are prob- ably

related to crustal thicke ning as

the resu lt of the collision with

the Lucon ia Bloc k. Hutchis on

(1996 a) sugges ts that Miocene

mag matism along the Cagay an

Rid ge does not continue into

Saba h. Ho wever, Bellon and

Rang in (1991) point out that theintrusive rocks of Mo unt Kinab alu

are also of calc-alka line anity,

wh ich may indicate that the

mag matism of the Cag ayan belt

does not extend west wards into

Sabah. The Late

Miocen ePlei stocene mag matic

arc of the Sulu Sea continues into

the Dent and Sampor na penin-

sula s. This mag matism is regard ed

as rela ted to subduction in the


27/33

Sulu Tre nch (Hutchison, 1996a;

Be llon and Rangi n, 1991; Rang in

and Silv er, 1991). The western

part of this sub duction sys tem is

bound ed by a


28/33

Fig. 6. A, The magmatic belt of Central Kalimantan during Eo-Oligo cene time. B, Schematicregional cross-secti on (modied from James, 1984).

1, Magmatic belt; 2, accretionary prism belt. Numb ers are K/Ar dates from volcanic rocks.


29/33

Fig. 7. A, Collision belt of North Kalimantan during Middle Oligocene time. B, Schema tic

cross-secti on. 1, Folded belt/collisi on zone; 2, magmatic belt. Numbers are Ka/Ar ages from

volcanic rocks.


30/33

Fig. 8. A, Magmatic belt of Kalimantan during Late Oligoce neMiddle Miocene and Late

Miocen ePleistocene times. 1, Late Oligoce neMiddle Miocene magmatic belt related to the

Eocene subducted slab; 2, Middle Miocen ePliocene magmatic belt related to subduction inthe Palawan Trench; 3, Late Miocen ePleistocene magma tic belt related to subduction in

the Sulu Trench. B, Schematic cross-secti on. Numbers are K/Ar ages from volcanic rocks.


31/33

NN W SSE sinist ral strike- slip fault

wh ich may be a north ward

extension of the Palu Koro Fault

(Fig. 8). Nor thward mov ement of

Celebes Sea Plate, as a conse-

quence of the collision of

Ban ggai-Sula with eastern

Sula wesi, may have ini tiated the

subd uction which for med the Sulu

Arc.

Relat ively large outcrops of

Plio -Pleistoce ne volcanics have

been mapped by Pieters et al.

(1987) in the Niewen huis

Mo untains and Maha kam region,howev er none of them have been

described. Gener ally they are

basal tic in composition and their

rock chem istry shows continental

basalt anity. Basaltic roc ks from

Sintang and Sabah display

relativ ely high magne sia conten ts,

indic ating a more pri mitive

compos ition, sugges ting the

invol vement of oceanic basem entin their formati on.

The val idity of the inte rpretation

of the geo dynamic his tory of

Kaliman tan and Bo rneo as

presented above may be subject to

revision when recent work by

Fuller et al. (1999) is taken into

account. Fuller et al. (1999) report

two signi cant co untercloc kwise

rotations of Borneo which are not

incorpor ated in current synth- eses.

A better und erstan ding of the

geodynam ic hist ory of Kalima ntan

and Bo rneo will resu lt from

furth er geo physical work,

espec ially in those

areas of Kaliman tan whe re

eld data is scarce or unavaila ble.

Acknowledgements

The present authors are grat eful

to R.C. Maury and H. Bello n,

Univ ersite de Bretagne

Occide ntale, Brest (France ), for

their commen ts on the

geochem istry. We also thank J.P.

Rampnou x, Univ ersite de Savoie,

Le Bou rget du lac (France ), for his

sugges tions concerning the

geody namic inte rpretation.

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