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

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 Ma. The Group 1 samples (ca. 205 Ma) have low SiO2 content (64–67 wt%) and low differentiation index values (DI = 75–76), and are slightly depleted in Ba, P, Sr, and Ti compared to the Group 2 samples (ca. 221 Ma and 213 Ma), which have high SiO2 content (74–75 wt%), 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 εHf(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 ± K-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.