Crystal fractionation of contaminated melts and re-melting of newly underplated basaltic crust generated Late Triassic andesitic and dioritic intrusions in the southern Yidun Terrane, SW China

Abstract

The genesis of andesitic and dioritic rocks in continental arcs is hotly debated, and proposed models include slab melting, juvenile mafic lower-crustal melting, mafic and felsic magma mixing, and mantle-derived melts that underwent a MASH (melting, assimilation, storage, and homogenization) process. The controversy comes from the possibility of their polygenesis and the ambiguity of bulk-rock isotope tracers, not to mention the processes of magma evolution. Here, we report on our investigations in the southern Yidun Terrane (a Triassic continental arc in SW China) into the petrogenesis of two stages of the dioritic Xiuwacu intrusions (phases 1 and 2), the mafic microgranular enclaves (MMEs) hosted by the phase 1 intrusion, as well as the andesitic Hongshan intrusions. We used U-Pb zircon dating, in situ zircon Hf-O isotope analyses, bulk-rock Sr-Nd isotope analyses, and trace element analyses of the zircons from different stages of the intrusions and the MMEs to shed light on their sources and mantle–crust interactions involved in their petrogenesis. Our results suggest that the Xiuwacu MMEs from the phase 1 intrusion crystallized from a melt. Their variable zircon Hf-O isotopic compositions (εHf(t) = +0.4 to +5.3 and δ18O = 4.80‰ to 7.79‰) indicate they were formed by the mixing of mantle-derived basaltic melts and felsic melts. The phase 1 intrusion have an origin similar to that of the MMEs, as indicated by their homogeneous zircon Hf-O isotopic compositions (εHf(t) = +1.3 to +3.9 and δ18O = 6.58‰ to 7.31‰), which fall within the range of the isotopic compositions of the MMEs. The discovery of several inherited zircons and a small recrystallized felsic microgranular enclave hosted by an MME reveals that the MMEs contain relatively more felsic components. Even so, the MMEs still have lower silica contents (62.1–63.4 wt%) than those of the phase 1 intrusion (63.4–70.5 wt%), and this indicates that direct magma mixing of basaltic and felsic melts generated the andesitic MMEs, whereas a crystal fractionation process is required for the dioritic phase 1 intrusion. The porphyritic phase 2 and Hongshan intrusions formed at ~216 Ma, after the phase 1 intrusion, and they had higher oxygen fugacities and crystallization temperatures than the phase 1 intrusion. Given the conclusions of previous researchers that slab breakoff probably occurred at ~216 Ma, we propose that the Xiuwacu phase 2 and Hongshan intrusions formed during the slab breakoff process, the former by partial melting of the newly underplated basaltic lower crust in the MASH zone, and the latter by differentiation of mantle-derived basaltic melts at relatively high pressures.