Petrogenesis of ultramafic rocks and associated ...

LITHOS ELSEVIER

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Lithos 35 (1995) 153-182

Petrogenesis of ultramafic rocks and associated chromitites in the Nan Uttaradit ophiolite, Northern Thailand B. Orberger”,“, J.P. Lorandb, J. Girardeau”, J.C.C. Merciera*d,S. Pitragool” “Laboratoire de P&-ologie Physique, Universite’Paris-7, lnstitut de Physique du Globe de Paris, 2 Place Jussieu, 75251 Paris Cedex 05, France / CNRS, URA N” 1093, France “Laboratoire de Mine’ralogie, Mus&an National d’Histoire Naturelle, UnitPassocike au CNRS N” 736, 61 rue de Buffon. 75005 Paris, France “Laboratoire de Pe’trologie Structurale; Universite Nantes, UFR Science et Tkchniyues, 2 rue de la HoussiniPre, 44072 Nantes Cedex 03, France “Present address: Lab. d’ktudes Physiques et Chimiques appliquies ci la Terre, PBle Sciences and Technologie. UniversitP de L.a Rochelle, Avenue Marillac, 17042 Lu Rochelle CJdex 01, France ‘Department of Geological Sciences, Faculty of Science, Chiang Mai University, Chiang Mai 50002, Thailand

Received4 March, 1993;revised and accepted 21 July, 1994

Abstract The ultramafic sequence and associated chromitites of the Nan-Uttaradit ophiolite in the northeastern part of Thailand have been studied in the field and by applying petrography and geochemistry to whole rock samples and minerals. The ultramafic rocks comprise irregulary shaped bodies of dunite, harzburgite, orthopyroxene-rich lherzolite and orthopyroxene-rich harzburgite, clinopyroxene-rich dunite and intrusive clinopyroxenite-websterite bodies. Three types of chromitite were distinguished. Type I chromitite lenses and type II layers which are hosted in orthopyroxenite in the northern part and in dunite in the central part of the ophiolite. Type III chromitite forms lenses or layers in clinopyroxenites in the central and southern parts of the belt. According to the modal and chemical composition the peridotites and orthopyroxenites are strongly refractory. They originated during different stages of interaction between percolating melts and peridotite. The chromitites of types I and 11,which are very rich in Cr (up to 68 wt.% Cr,OX), crystallized from a boninitic parental magma under highly reducing conditions in the northern part and moderate oxygen fugacities (FMQ) in the central part of the ophiolite. The chromitite of type III which are characterized by the highest Fe3+ /(Fe”+ + Cr + Al)-ratios, and hosted in intrusive clinopyroxenite-websterite-rocks, cumulated from a CaOrich transitional boninitic melt underflS conditions around FMQ.

1. Introduction Ophiolites are important to our understanding of magma generation and magma extraction from the mantle. Both the pillow lava and the upper mantle section of ophiolites indicate their derivation from different magma generations (Beccaluva and Serri, 1988; Ohnenstetter et al., 1990 and references therein). The oldest component of ophiolite sequences is usually a MORB-type tholeiite where plagioclase has crystal0024-4937/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIOO24-4937(94)00041-7

lized first. It is overlain by various highly magnesian andesites and basalts where pyroxene has crystallized before plagioclase. These rocks show the typical highfield strength element (HFSE) depletion of island-arc magmatism. Some rocks, which have crystallized orthopyroxene or clinopyroxene and highly chromiferous Cr-spine1 first, show boninitic affinities. Due to the presence of the rocks formed from such magmas, most authors agree that ophiolites were likely formed in mar-

B. Orberger et al. /Lithos 35 (1995) 353-182

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ginal basins above subduction-zones rather than at midocean ridges (e.g. Elthon, 199 1; Pearce, 199 1) . Intrusive ultramafic bodies cross-cutting the mantle sequence and the mantle/crust boundary have been recognized in several ophiolite complexes (e.g. Troodos: Benn and Laurent, 1987; Oman: Ernewein et al. 1988; Benn et al., 1988; Appalachian ophiolites, Canada: Laurent and Hebert, 1989). They are characterized by the crystallization of clinopyroxene and/or orthopyroxene before olivine and plagioclase. It has been suggested that these bodies may represent cumulates segregated from the low-Ti magmas that produced the upper pillow lavas (e.g. Thy and Moore, 1988; Ohnenstetter et al., 1990; Thy and Xenophontos, 1991). Orthopyroxenite and clinopyroxenite bodies of this type also occur within in the presently studied lower ophiolite sequence of the Nan Uttaradit (NU) belt in Thailand. They host different types of chromitite bodies which are unusual rich in chromium. The present paper presents a detailed mineralogical and geochemical study of both, the chromitites and their associated ultramafic rocks. Like many ophiolites related to continent-

continent collisions, the NU ophiolite is strongly dismembered and hydrothermally altered. The principal aim of the present study was therefore to identify the protoliths of the ultramafic rocks by using whole rock and mineral major and trace element data. Chromspine1 compositions in conjunction with silicate paragenesis were used to constrain the compositions and redox conditions of the different generations of melt that percolate the NU ophiolite.

2. Geology and tectonic setting The Nan-Uttaradit (NU) ophiolite belt, in the northeast of Thailand, is a 10 km large and 150 km long NESW trending suture zone separating two major continental cratons (Fig. 1a) : the Shan-Thai craton to the West and the Indosinian-Chinese craton to the East (Bunopas and Vella, 1978; Thanasuthipitak, 1978; Sengor, 1979; Ridd, 1980; Huang, 1984; Barr and Macdonald, 1987; Cooper et al. 1989; Hutchison, 1989; Panjasawatwong and Crawford, submitted). It extends

a CHINA

CHINESE CR AT ON

17O30’N

Fig. 1. Geotectonic situation of the Nan Uttaradit ophiolite. a. general geotectonic Uttaradit area, northeastern Thailand (modified after Thanasithapak et al., 1978).

framework

of southeast Asia. b. geologic

map of the Nan

B. Orberger et al. /Lithos 35 (1995) 153-182

from east of Mae Charim in the north to about 20 km south of the Sirikit reservoir, about 30 km east of Uttaradit and has probably its prolongation near Prachin Buri near the Cambodian border (Fig. la). Mafic and ultramafic ophiolitic rocks appear as tectonized slices within sediments. These sediments consist of sandstones, slates, shales and conglomerates of Permo-Triassic and Carboniferous (Fig. 1b) age in the west, Siluro-Devonian in the southeast and Jurassic in the eastern and north-eastern parts (Baum et al., 1970). Extensive faulting and thrusting during emplacement have dismembered the Nan-Uttaradit ophiolite and obscured the relationships with the host sediments. The geotectonic setting in which the NU ophiolite originated is still debated. Helmcke ( 1985) questioned whether it represents a remnant of Paleotethys seafloor or whether it was formed to the east of Gondwana or within a marginal sea which was situated off Paleoeurasia. A continent-continent collision model has been proposed by many authors for Thailand. Bunopas and Vella (1978), Barr and Macdonald (1987) and Barr et al. ( 1990) suggested a westward subduction beneath the Shan-Thai craton whereas an eastward subduction under the Indosinian craton was proposed by Beckinsale et al. (1979). A third model considers a pair of subduction zones, one dipping to the west and the other dipping to the east (Bunopas and Vella, 1978; Thanasuthipitak, 1978; Cooper et al., 1989; Panjasawatwong and Crawford, submitted). The latter authors suggest that a northeast-dipping subduction of oceanic crust, initiated beneath the Indochina craton, later changed to southwest-dipping subduction beneath the Shan-Thai craton. There is also a considerable uncertainty about the timing of the continent-continent collision. Estimated ages range from pre-Late Permian (Helmcke, 1985)) Middle to Late Permian (Barr and Macdonald, 1987), Permo-Triassic (Thanasuthipitak, 1978; Cooper et al., 1989; Barr et al., 1990)) Triassic (Beckinsale et al., 1979) and Late-Triassic (Bunopas and Vella, 1978; Sengiir, 1979; Hutchison, 1989). The contrasting opinions on the genesis of the Nan-Uttaradit suture are due mainly to the poor exposure of the ophiolite rocks and the effects of tectonic events during and after the collision, i.e. extensional collapse of the overthickened crust during the late Triassic and Late-Triassic to Cenozoic transcurrent movements (Cooper et al., 1989). However, whatever the chosen model is, the formation of the Nan-Uttaradit ophiolite above a subduction zone

155

is confirmed by geochemical studies of the volcanic rocks. These comprise oceanic-island basalts, back-arc basalts and andesites and island-arc basalts and andesites of Carboniferous to Permo-Triassic age (Panjasawatwong and Crawford, submitted). 2.1. Lithology of the NlJ ophiolite Ultramafic rocks crop out all over the belt, whereas mafic rocks, in particular the gabbroic sequence, are mostly exposed only in the central part (CIG-company, unpubl. data). East-west profiles across the belt show that the ophiolite consists of tectonic slices thrusted to the east over sedimentary rocks of uncertain age. Around Pak Nai, the nearly NE-SW-trending thrust contacts and fault zones are often characterized by silicified serpentinite lenses, while massive magnesite occurs in the northern (Rae Nan) and central parts (Pak Nai) of the belt. Fig. 2 displays a schematic cross-section through the ophiolite. Although the main units are often dismembered, primary magmatic and sedimentary contacts are

Fig. 2. Schematic synthetic section of the Nan-Uttaradit

ophiolite.

1.56

B. Orberpr

et al. /Lithos

preserved locally. The ophiolite can be subdivided into ( 1) an ultramafic unit, consisting of peridotites and variably amphibolitized pyroxenites, both containing chromitite orebodies and (2) a mafic unit, comprising more or less amphibolitized layered and isotropic gabbros hosting small intrusive granitic bodies and dolerites. The dolerites are massive and do not form a well defined sheeted-dike complex. The mafic sequence is crosscut by different types of dikes: coarse-grained orthopyroxenites, plagioclasite (anorthite-rich with large pyroxene crystals) in the gabbroic rocks and, finegrained epidotite and late basalts of various width (5 50 cm) in the dolerites. Pillowed lavas with their sedimentary cover, comprising bedded red, green and black cherts, rarely rest upon the mafic cumulates. Ultramafic rocks appear in the central part of the belt as three tectonic slabs, which are more or less connected. Our samples were collected in the area of Mae Charim and Rae Nan in the north, Pak Nai in the central part of the belt, and Prachin Buri in the south (Fig. la). In the central part, the westernmost slab that borders the Permo-Carboniferous (?) sediments mainly comprises serpentinized harzburgites and minor pyroxenites. The central slab consists of highly serpentinized harzburgites, orthopyroxenites and dunites. The largest slab which crops out south of Pak Nai consists of more or less amphibolitized pyroxenite layers and sheets (up to 300 m thick) within serpentinized harzburgite and orthopyroxenite. The pyroxenites form centimetre- to metre-sized sills which are generally separated from the host peridotites by thrust contacts hiding their intrusive relationships. An even larger clinopyroxenite body, up to 400 m thick, is clearly intrusive within the mantle/crust boundary (Fig. 2). It is in contact with peridotites to its base and with layered gabbro at its top. The latter likely marks the bottom of the oceanic crust. Both the clinopyroxenite and the gabbros are partly amphibolitized and foliated because of regional metamorphism related to the continental collision. The ultramafic rocks also contain three main types of chromitites (Figs. 2, 3) : -type I chromitites are lenses and hosted in peridotites and orthopyroxenites (Fig. 3a). They were found essentially in two parts of the ophiolite belt. A large chromitite lens of about 20 m length and 10 m width is located near Mae Charim. It is separated from the surrounding peridotite by an EW-trending shear zone. Several chromitite lenses of centimetric to metric size

35 (1995) 153-182

occur in the central part (Pak Nai). Mining work has obscured the relationships between these chromitite lenses and their hosts. Nevertheless, some small chromitites lenses show a thin serpentinized dunitic envelope. -type I1 layered and banded chromitites are most abundant. They are hosted in peridotites and orthopyroxenite all over of the ophiolite. The largest layers (30 cm thick) have been found at Rae Nan (Figs. 1b, 3b). Multiple thin chromitite bands, several centimeters in thickness crop out south of Pak Nai (Figs. 1b, 3~). In both cases, the chromitite orebodies grade into disseminated chromite (50% Cr-spine], 50% silicate) in the host peridotites. En-echelon chromitite veins crosscut the hostrock (Fig. 3b). -type III chromitites are lenses of different sizes within the lower part of the large intrusive clinopyroxenite body discovered in the Pak Nai area. They display sharp and locally secant contacts with the host pyroxenite (Fig. 3d). The largest bodies of metre-size are folded and affected by the same metamorphic foliation as the host pyroxenite. The smallest discontinuous thin (cm-size) bands/layers are orientated parallel to the pyroxenite foliation (Fig. 3d).

3. Analytical methods Sixty rock samples have been investigated in polished thin sections by conventional microscopic techniques, using both transmitted and reflected light. About fourty peridotites and sixteen pyroxenites have been analysed for major and minor elements as well as for a number of trace elements by X-ray fluorescence spectrometry. These samples were collected at distances from massive magnesite or silicified serpentinites. A first set of major element analyses was collected at the Institut fur Mineralogie und Lagerstattenlehre at the Technical University of Aachen, Germany, using Phillips Typ PW 1400. A second set of analyses including some minor and trace elements (Cr, Ni, Co, SC, V, Zn, Sr, Ba, Ce, Nd, Zr, Nb) was performed at the School of Earth Science, University of Melbourne, Australia, using an ARL 8420 spectrometer. Major and minor elements as well as trace elements were analysed using glass beads prepared according to the method described by Haukka and Thomas ( 1977). Representative analyses of peridotites and pyroxenite subtypes

B. Orberger

et al. /Lithos

35 (1995) 153-182

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Fig. 3. Photographsof chromitites:a. type I chromititepods with dunitic envelope (Pak Nai); b. type II layered chromitite in orthopyroxenite (Rae Nan); c. type II chromitite bands in dunite (Pak Nai); d. type 111chromitite lens in intrusive clinopyroxenite.

as well as detection limits of the analytical methods are shown in Tables 1 and 2. Phase compositions were determined with an ARLelectron microprobe at the Institut fiir Mineralogie und Lagersttittenlehre, RWTH Aachen, Germany, using acceleration voltage of 20 k, a current beam of 20 nA and a counting time of 20 s. Another set of data have been obtained with fully automated CAMEBAX-electron microprobes “Microbeam” at the BRGM (OrlCans, France) and CAMPARIS (Universitt Paris VI, France) at 15 kV and 40 nA with a counting time of 20 s. It was not possible to analyse all phase compositions in all the rock types because of the too strong hydrothermal alteration and metamorphism. Thus, a special attention has been paid to olivine, clinopyroxene and chromite in order to trace variations of mg ( 100 Mg/Mg + Fe in atoms) and Cr ( 100 Cr/Cr + Al in atoms) numbers. Representative analyses are reported in Table 3.

4. Petrography 4. I. Peridotites and orthopyroxenites Modal compositions Peridotites and orthopyroxenites collected from shear zones (e.g. Pb 1, Pb 6) are strongly serpentinized as demonstrated by the loss on ignition values (L.O.I.) of whole-rock analyses (Table 1). Due to the high degree of serpentinization a metamorphic texture could not be recognized. Spine1 is the less altered mineral. It is slightly transformed into ferrite-chromite. Olivine is replaced by hour-glass chrysotile-lizardite, sometimes coexisting with brucite in serpentinized dunites north of Pak Nai. Orthopyroxene is bastitized. Fine-grained magnetite is abundant in the serpentine matrix or forms overgrowths on Cr-spinels. Even clinopyroxene are transformed into serpentine or Ca-amphibole in the Rae Nan and Prachin Buri samples. The degree of serpen-

1%

Table

B. Orberger et al. / Lithos 35 (I 995) 153-I 82

1

Representative major and trace element compositions of peridotites and orthopyroxenites.

Detection limits for minor elements

(wt.%):TiO,:

0.02; CaO: 0.02; Na,O:

0.50); detection limits of trace elements (ppm): Cr, SC, V, Co, Ni: 3; Zn, Zr, Nb, 2; FelO, stands for total iron. Mg# = lW*Mg/(Mg+Fe) to the CaO loss in most of the pyroxene-rich lherzolites, original CaO contents were recalculated by assuming a constant Sc/Ca-ratio Al,O, ratios (prior to CaO-loss) is reported as CaO/Al,O: Opxte

Opxte: orthopyroxenite:

Opxte MC 38

Opxte

Lherz.

Lherz.

PB6

MC40

PB 1

PB 3

shearzone

shearzone

shearzone

48.12