ABSTRACT. Tumbarumba gemfield in the Snowy Mountains basalt province, NSW, yields corundums, zircons and garnet, corroded by magmatic effects and abraded by alluvial transport. Sub-basaltic contours suggest present drainage profiles mimic Miocene sub-basaltic leads. Six types of corundum were identified. Blue, green, yellow (BGY) zoned sapphires (80%) contain ferrocolumbite as a main mineral inclusion and exhibit variable Fe 2 O 3 /TiO 2 and low Cr 2 O 3 /Ga 2 O 3 (<1). Two sub-types differ in colour absorption spectra, one being unusual in lacking the typical Fe 2+ -Fe 3+ charge transfer effects found in such sapphires. Related trapiche-like corundums (5%) show higher Cr 2 O 3 /Ga 2 O 3 , possibly due to Fe-Ti oxide exsolution. Vari-coloured, diffuse-zoned and pale blue sapphires (10%) have higher Cr 2 O 3 /Ga 2 O 3 and colour absorption characteristics intermediate between BGY sapphires and pink to red corundums with elevated Cr 2 O 3 /Ga 2 O 3 . The BGY and trapiche-like sapphires are considered magmatic, the intermediate sapphires magmatic-metasomatic (possibly through interactions with Cr-bearing serpentinite bodies) and the pink to red corundums metamorphic in origin. Zircons include low-to high-U types. The latter show {100}-{110} prism combinations (unusual in eastern Australian zircons) and suggest incompatible element enriched parental melts. The magmatic sapphires and zircons (U-Pb age 23 Ma) crystallised in deep evolved salic melts, before transport in basalt. Magmatic-metasomatic sapphires contain zircon inclusions with both older inherited U-Pb ages (up to 903 Ma) and younger magmatic UPb ages . Basalts represent little evolved undersaturated melts (basanites and alkali basalts), and minor near-saturated transitional melts (olivine basalts). Most generated from garnet peridotite sources, but some from spinel peridotite sources. Mantle normalised incompatible multi-element patterns suggest Oceanic Island Basalt (OIB) melts interacted with amphibole (+ apatite) veined mantle. A sapphire and zircon-bearing basalt, also carries kaersutitic amphibole, apatite, alkali feldspar, titanian mica and titanian magnetite xenocrysts from a veined metasomatised source. Olivine micro-dolerite in a plug resembles the Cainozoic basalts in freshness, but its distinct trace element pattern and Early Devonian K-Ar age (400 Ma) indicate an earlier unmetasomatised spinel peridotite source. The Tumbarumba field evolved through explosive gem-bearing basaltic activity between 27-15 Ma and peaked in basalt lava activity. Interactions of basaltic melts with amphibole-rich mantle, serpentinite bodies and metamorphic corundum deposits combined to generate multi-modal gem suites.
The Shyok Suture Zone separates rocks in the Asian plate from rocks in the Kohistan-Ladakh island arc.In Baltistan, this suture has been reactivated by the late 'break-back' Main Karakorum Thrust (MKT). The P-T histories of metamorphic rocks both north and south of the MKT have been determined in an effort to place constraints on the tectonic history of this zone. The terranes north and south of the MKT have different, unrelated metamorphic histories. Rocks from the Kohistan-Ladakh island arc south of the MKT have undergone a static low-P (2-4 kbar, c. 500" C) thermal metamorphism. The P-T paths and metamorphic textures of these rocks are consistent with metamorphism due to emplacement of plutonic rocks into the island arc. This metamorphism pre-dates folding and deformation of these rocks. Rocks in the Karakorum Metamorphic Complex, north of the MKT, have experienced a complex deformational and metamorphic history. Prograde metamorphic isograds have been deformed by subsequent southverging folding and by gneiss dome emplacement. However, decompression metamorphic reactions occurred during nappe emplacement. Higher pressure rocks are associated with higher level nappes, creating an inverted pressure metamorphic sequence (8-9-kbar rocks over 5-6-kbar rocks). There is little variation in temperature with structural level (550-625" C). These t\ko different terranes have been juxtaposed after metamorphism by the late south-directed MKT.
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