"Rock of the Month # 39, posted for September 2004" --- The illustrated sample is 37x21 mm in area, roughly 0.035 mm thick, a translucent sliver of rock 7.7 cm² in area, which weighs roughly 0.08 g (one 360th of an ounce), known as a polished thin section. This type of prepared sample, pioneered in the late 19th century, enables a trained microscopist to identify the constituent minerals in a rock, note the textures within, provide a standard name for the rock type, and speculate on its origins and significance.

GCW sample 2428. Digital image, 27 October 2004. This sample was donated by Rob Davies who, together with Roy Lindsay, mounted a climbing expedition to the area in 1998. The rock was collected from near the summit of the massif (Low's Peak, 13,455 feet, 4,101 m), far above the tree line which lies near 11,000 feet (3,353 m).

Diorite is a common variety of plutonic igneous rock, found in many parts of the world. It is the intrusive equivalent of the common lava type andesite, and both rocks are intermediate in composition between iron and magnesium -rich rocks (such as gabbro and basalt) and more silica-rich types (granite, rhyolite). This particular sample is quite fresh (not greatly altered since it crystallized from its parent magma, not by chemical or physical processes underground, nor by weathering since uplift and erosion brought it to the surface). A classic diorite is rich in sodic plagioclase feldspar, speckled with dark silicates such as hornblende and augite. This particular example is composed of some two-thirds plagioclase feldspar by volume, plus abundant hornblende, quartz and biotite mica, plus traces of iron-titanium oxides, apatite, epidote and zircon. The hornblende occurs as large crystals referred to as phenocrysts, in a finer-grained groundmass enriched in smaller crystals of feldspar and quartz. Such a texture of large phenocrysts in finer matrix) is referred to as porphyritic. The feldspar displays complex twinning and ubiquitous oscillatory compositional zonation, in concentric bands surrounding each crystal core. This is indicative of crystal growth in a magma chamber under disequilibrium conditions. These conditions might involve episodic loss of magma through eruption, and/or the mixing of different batches of magma in the chamber. Stocky green hornblende prisms and dark brown flakes of biotite mica are visible in the photograph.

Kinabalu is situated in Sabah province of Malaysia, on the northern margin of the giant island of Borneo. The region is tectonically active, the scene of young (Neogene) plutonism and local, ongoing volcanism (e.g., Hamilton, 1979). The upland of Kinabalu is underlain by a large (circa 40x20 km) granitoid batholith, dated at about 9 Ma (nine million years, dating to the upper Miocene period: Kosaka and Wakita, 1978). The exposed part of the Kinabalu batholith, some 155 km² in area, consists largely of a hornblende quartz monzonite (Vogt and Flower, 1989), a rock type with less plagioclase and more alkali feldspar than this sample. The genesis of the magmas is influenced by crustal assimilation from subducting lithosphere from the South China Sea plate. Vogt and Flower (1989) published a detailed map of the Kinabalu area at roughly 1:85,000 scale, showing the four topographic highs (Alexander Peak, Victoria Peak, Low's Peak and South Peak).

Economic concentration of metals may occur within and marginal to such igneous masses within the Earth's upper crust. The Mamut porphyry copper (-gold) deposit is an example of mineralization developed in apophyses of the batholith and its host rocks: late Cretaceous to early Tertiary sediments cover the northern part of Borneo (Kosaka and Wakita, 1978). Immiscible sulphide globules (pyrrhotite and chalcopyrite) have been noted in a thin, late sheet-like intrusion cutting the plutonic rocks of this Miocene complex (Imai, 1994).

The mountainous topography and consequent attraction for mountaineers is summarized by Bueler (1970). The massif reveals geologically young igneous rocks of broadly granitic composition, brought rapidly to the Earth's surface in a tectonically active part of the world. The Miocene, 9 Ma date for emplacement of the Kinabalu batholith can be compared with the Cuillin gabbros of Skye, Scotland (Paleocene, 59 Ma) or the Tuolomne intrusive suite of Yosemite, California (late Cretaceous, 89-81 Ma).

References

BUELER,W (1970) Mountains of the World, a Handbook for Climbers and Hikers. The Mountaineers, Seattle, 279pp., pp.239-243.

HAMILTON,W (1979) Tectonics of the Indonesian Region. USGS Prof.Pap. 1078, 345pp. plus 1:5,000,000 scale map.

IMAI,A (1994) Sulfide globules associated with a felsite intrusion in the Mount Kinebalu quartz monzonite, Sabah, east Malaysia: sulfide melt immiscibility in a highly silicic melt. Econ.Geol. 89, 181-185.

KOSAKA,H and WAKITA,K (1978) Some geologic features of the Mamut porphyry copper deposit, Sabah, Malaysia. Econ.Geol. 73, 618-627.

VOGT,ET and FLOWER,MFJ (1989) Genesis of the Kinabalu (Sabah) granitoid at a subduction-collision junction. Contrib.Mineral.Petrol. 103, 493-509.