Petrogenesis of the early Jurassic Ora batholith in south‐western Cambodia

• Published in: Geological journal (Chichester, England) England), 2022-08, Vol.57 (8), p.3230-3250
• Main Authors: Zeng, Xiang‐ting, Cong, Feng, Zhu, Hua‐Ping, Shi, Mei‐Feng
• Format: Article
• Language: English
• Subjects: Aluminum oxide; Asthenosphere; Batholiths; Biotite; Cambodia; early Jurassic; Feldspars; fractionated; Granite; Isotopes; Jurassic; Lava; Lithosphere; Magma; Melting; Ocean circulation; Ora granites; Orogeny; Palaeo‐Tethys; Paleoceanography; Petrogenesis; Phosphorus pentoxide; post‐collisional; Radiometric dating; Rocks; Silica; Silicon dioxide; Thickening; Titanium dioxide; Upwelling; Zircon
• ISSN: 0072-1050; 1099-1034
• DOI: 10.1002/gj.4474

The Ora batholith in south‐western Cambodia is situated in the western part of Indochina and east of the Inthanon–Bentong–Raub Palaeo‐Tethys suture and thus bears significant constraints for the equivocal crustal generation of the Palaeo‐Tethys orogenic belt in Mainland South‐east Asia. The Ora batholith consists of monzonitic granite (Group 1), alkali‐feldspar granite (Group 2), and biotite granite (Group 3), and they yield similar Early Jurassic ages of 186.0–199.7 Ma. Group 2 are high‐silica (SiO2 > 75 wt%) rocks with lower TiO2, Al2O3, FeOtot, MgO, CaO, P2O5, Eu, Ba, Sr, P, and Ti contents compared to those of the other two groups. Group 2 and 3 are classified as fractionated‐type granitoids, whereas Group 1 is unfractionated granite as the complementary residual of the fractionated rocks. All groups have positive εHf(t) values from +2.29 to +8.83, suggesting they were cogenetic and derived from partial melting of the lower crust with the addition of asthenospheric mantle material. Integrating with other granitoids in this region, we suggest the Ora granitoids belong to I‐type granitoids. Our petrogenetic and zircon U–Pb age data suggest the Ora granitoids were related to the Early Jurassic post‐collisional magmatism that resulted from crustal thickening and mantle lithospheric delamination due to a denser crustal root than the underlying mantle lithosphere after the Palaeo‐Tethys Ocean closure and collision between the Sibumasu and Indochina terranes. Then the following upwelling of the asthenospheric mantle not only heated and caused partial melting of the lower crust and mantle wedge, but also supplied asthenospheric material to generate the high‐εHf(t) magma. (a) Subduction phase of the Paleo‐Tethys Oceanic crust to the Indochina Block during Permian to Middle Triassic. (b) End of the subduction of the Paleo‐Tethys Ocean and collision of the Sibumasu Block with the Indochina Block accompanied by the detachment/break‐off of the eclogitized slab as the density change. (c) After Paleo‐Tethys Ocean closure, the collision between the Sibumasu and Indochina terranes led to crustal thickening and mantle lithospheric delamination due to a denser crustal root than the underlying mantle lithosphere (Kay & Mahlburg Kay, 1993). Then upwelling of the asthenospheric mantle would rise into the crust, where they further induce large‐scale crustal melting and magma reservoirs formation, resulting in granitic magmatism silicic magmas which would rise from their deep