The mount Tawai Peridotite, north Borneo

Abstract

The Mount Tawai peridotite is a batholith, elongate north-south, situated between the Kinabatangan and Labuk rivers in the North Borneo ultrabasic belt. The surrounding country rocks are Upper Cretaceous and Tertiary sediments and volcanics which strike predominantly E.N.E. They are separated from the peridotite by a fault breccia which contains inclusions of metamorphic rocks. The batholith comprises at least three partially separated tectonic units, the largest of which is the Main Tawai block. Theouter parts of the tectonic units exhibit a crude gneissose foliation and peripheral serpentinization. The batholith is composed mainly of harzburgite with scarce dunite, pyroxenite and gabbro phases. The dunite occurs in lenticular pods which dip steeply westward in the Main Tawai block. Some of the dunite lenses contain thin chromite bands showing evidence of rudimentary gravity stratification. The harzburgite is composed mainly of forsterite and enstatite with only minor endiopside. The alumina content of the enstatite varies from 1.3 to 7.4 per cent and that of the endiopside from 2.9 to 7.2 per cent. The dunite bandscontain forsterite and chrome spinel, the composition of the latter varying from cr(_65.5) AL(_32.2) (Mg(_68.1)) to cr(__32.6) AL(_66 .9) (Mg(_63.8)) on Thayer's (1946) shortened formula. The gabbroic rocks are irregularly distributed within the batholith occurring mainly as tectonic inclusions. Both the ultrabasic and basic rocks are considered to have been derived from the same parent magma. The absence of anorthite from the ultrabasic assemblages is explained by an initial phase of differentiation at very high temperature and pressure which suppressed the precipitation of plagioclase but favoured the introduction of alumina into the pyroxenes. During the initial phase of differentiation only olivine, enstatite, endiopside, and spinel were precipitated, but there must have been frequent but short-lived intervals in which only olivine and spinel were precipitated; these are thought to have accumulated in hollows and channels on the floor of a crystal pile. A systematic increase of alumina in the pyroxenes and spinels of the Main Tawai block has been traced and is attributed to a crystallisation sequence. A gabbroic phase is thought to have been precipitated on top of the ultrabasic crystal pile following a drop in temperature which resulted in the precipitation of plagioclase and the lowering of the Mg/Fe ratio in the pyroxene and olivine. A calculation of the average composition of the rocks of the North Borneo belt is in close agreement with the average mantle composition suggested by Ringwood (1959). This evidence together with the high temperature and pressure conditions needed to explain the ultrabasic assemblages has led to the conclusion that the parent magma of the Mount Tawai complex was derived by fusion of upper mantle material. After or during the final stages of differentiation the gabbroic fraction was injected into the crust creating a pre-heated path up which the hot, already-differentiated peridotite rose as a series of almost crystalline units. During this stage a gneissose folliation developed roughly parallel to the sides of the intrusive units. Thermal metamorphism of the country rocks to garnet amphibolite fades accompanied the process. After cooling beneath the surface the intrusive units were serpentinised and emplaced in their present position by faulting, which also disrupted the thermal metamorphic aureole. Late lime bearing solutions, deposited calc-silicate veins and altered some of the gabbro to rodingite. The final emplacement had profound effects on the drainage system and occurred in late or post Pleistocene.