Petrogenesis, U-Pb zircon geochronology and tectonic evolution of the Malaysian granite provinces in the Southeast Asian tin belt

The Malaysian granitoids form the backbone of the Malay Peninsula and have long been recognized as composed of two distinct granitic provinces separated by the Bentong-Raub suture zone:
Early Permian to Late Triassic Eastern Province (Indochina – East Malaya) with mainly “I-type” hornblende-bearing granitoids, associated with Cu-Au deposits, and subordinate hornblende-free pluton roof-zones hosting limited Sn-W deposits; and
Late Triassic Main Range Province, western Malaysia (Sibumasu) with mainly “S-type” hornblende-free granitoids, associated with Sn-W deposits, and subordinate hornblende-bearing granitoids.
Field observations and new geochemical data suggested that the division of the Eastern Province and Main Range granitoids using Chappell and White’s (1974) I-S classification could be problematic, as there is a large degree of overlap between the two granitic provinces in terms of lithology, mineralogy and metallogenic affinity. The Main Range granitoids are more fractionated than the hornblende-bearing Eastern Province. Although the two granitic provinces were emplaced into different continental terranes, both granitic provinces exhibit common trace element geochemistry in the enrichment of high field strength elements (HFSE) and rare earth elements (REE) compared to typical Cordilleran I-S granites. Such enrichment is interpreted as an inheritance signature from the protoliths. The Kontum massif (an analogue of Indochina lower continental crust) comprises intraplate ortho-amphibolites and para-gneisses, which could serve as two hypothetical source end-members for the Malaysian granitoids. The model suggests that the geneses of the parental magmas of the Eastern Province and the Main Range Province were related to hybridization of melts derived from protoliths, geochemically and isotopically similar to these two source end-members, but in differing proportions. The fact that the granites from the two granitic provinces are so similar compositionally and metallogenically, suggests that similar protoliths were involved in their source. The incorporation of sedimentary-sourced melt makes the Main Range granitoids transitional I/S-type in nature, but this is unlikely to be true for the less evolved Eastern Province fractionated I-type granitoids. The hybridization of igneous- and sedimentary-sourced melts, and granite fractionation promotes Sn metallogenesis in the Main Range granitic province.
Previous ages were obtained using whole rock Rb-Sr and biotite K-Ar geochronology in the 1970s and 1980s, dating methods that almost certainly do not accurately represent the crystallization age of granites. New ion microprobe U-Pb zircon ages are presented that provide new temporal constraints for the Malaysian granitic magmatism. Eastern Province granitoids have U-Pb zircon ages that range from 289 to 220 Ma, while Main Range Province magmatism is constrained between 227 and 201 Ma. A progressive westward younging trend is apparent across the Eastern Province, but becomes less obvious in the Main Range Province. In addition, the U-Pb zircon analysis of the Malaysian granitoids suggests that both granitic provinces have Cambro-Ordovician and Mesoproterozoic inheritance signatures, which match the ages of the Kontum intraplate ortho-amphibolites and para-gneisses, the two source end-members of the suspected Indochina basement.
Two different tectonic models have been suggested to explain the formation and the emplacement of the Malaysian granitoids. Both models involve an east-dipping subduction zone during the Early and Mid-Triassic with Palaeo-Tethys lithosphere rolling back along the Bentong-Raub suture zone to produce westward younging ages in the Eastern Province granitoids. The first model (modified after Searle et al. 2012) suggests the younger Main Range granitoids were produced by another Late Triassic – Cretaceous east-dipping (Neo-Tethyan) subduction to the west of Sibumasu, after the Sibumasu – East Malaya collision. The transitional I/S-type geochemistry of the Main Range granitoids was caused by the partial melting of the more heterogeneous Sibumasu basement. The second model (Oliver et al. 2014) suggests the younger Main Range granitoids were produced by the westward underthrusting of Indochina crust of East Malaya beneath Sibumasu along the Bentong-Raub suture zone after the continental collision. In this model, the source of the Main Range granitoids was the pre-collision I-type Eastern Province granitoids. The second model is less likely, as no geological evidence for such underthrust is found in the Malay Peninsula.

Keywords: tin granite
Malaysian granite
Malay Peninsula
I-type
S-type
Southeast Asian tin belt
zircon dating
Advisor: Searle, M
Robb, L
Whitehouse, M