Generation of leucogranites via fractional crystallization: A case from the Late Triassic Luoza batholith in the Lhasa Terrane, southern Tibet

Abstract

Abstract Leucogranites are important in understanding intracrustal differentiation and regional tectonic evolution, but how these rocks form remains a matter of much debate. The Luoza batholith in the central Lhasa subterrane provides an important opportunity to address these issues as this batholith consists of normal calc-alkaline S-type granodiorites (Group 1) and highly fractionated S-type leucogranites (Group 2). Laser Ablation Inductively Coupled Plasma Mass Spectrometry zircon U-Pb dating indicates that these rocks were emplaced at ca. 221–205\u202fMa. The Group 1 samples (ca. 205\u202fMa) have low SiO2 content (64–67\u202fwt%) and low differentiation index values (DI\u202f=\u202f75–76), and are slightly depleted in Ba, P, Sr, and Ti compared to the Group 2 samples (ca. 221\u202fMa and 213\u202fMa), which have high SiO2 content (74–75\u202fwt%), high differentiation index values (91–94), and are significantly depleted in Ba, P, Sr, and Ti. Samples from Group 1 and Group 2 exhibit similar zircon eHf(t) values (−15.6 to −1.4), both of which are significantly higher than melts from the pure ancient basement in the central Lhasa subterrane (−20.5 to −10.6). The Group 1 samples can be interpreted as having been derived from the partial melting of metagreywacke within the ancient basement in the central Lhasa subterrane with contributions from mantle- or juvenile- crust-derived components, whereas the Group 2 samples may have formed via varying degrees of fractional crystallization of different minerals (e.g., plagioclase\u202f±\u202fK-feldspar, biotite, zircon, allanite/monazite, titanite, and apatite) from the parental magmas, represented by the granodioritic samples in Group 1, in shallow crustal magma chambers. In combination with sedimentary records, the high-temperature and low-pressure conditions indicated by the Group 1 samples can be geodynamically linked to a back-arc extensional setting in response to the southward subduction of the Bangong-Nujiang Tethys Ocean seafloor. Our work implies that fractional crystallization is likely a feasible mechanism for the development of leucogranites and for explaining the compositional diversity of the granite during intracrustal differentiation.