Continental crustal growth in collision zones on the Northern Tibetan Plateau

Abstract

The continental crustal growth has been a great interest to geoscientists and its importance is self-evident. Any models trying to reveal how it works must be able to explain 1) how the continental crust gains its andesitic bulk composition with juvenile isotope signatures; 2) how the volume of the crust increases episodically. Granitoids with the juvenile isotopic signatures in the collision zones provide the special insights into the nature of the continental crust and a unique opportunity to investigate the mechanism of crust growth. This thesis reports results from major and trace element data, whole rock Sr-Nd-Hf-Pb isotopic data, zircon geochronology and in situ Hf isotopes of granitoids as well as, if any, the enclosed mafic magmatic enclaves (MMEs) from the three ancient collision zones crossing the Northern Tibetan Plateau.
I-type granitoids and their MMEs from the East Kunlun Orogenic Belt (EK) in the Northern Tibetan Plateau are dated as 250 Ma years old. They are cal-alkaline in nature with compositions resembling the bulk continental crust (BCC). Whole rock Sr-Nd-Pb-Hf isotopes reveal that they are products of partial melting of the ‘trapped’ subducted oceanic crust at the onset of the collision and the MMEs are the aggregated early cumulates in the parental magma rather than the mafic endmember involved in the magma mixing as previously suggested.
I- and S-type granitoids from the Qilian Block (QB) further north in the Great Plateau are dated as 450 Ma. Their lithological and geochemical heterogeneity and isotopic changes with time are inferred to correspond to the collision. The abundant inherited zircons with ages as old as Archean reveal the presence of the Archean basement underneath the Qilian Block and indicate that the Qilian Block may have been a micro continent during its drift in the ancient ocean.
Granitoids and their enclosed cumulates from Kekeli Batholith further north in the plateau are 500 Ma years old. They have decoupled whole rock Nd and Hf isotopes and discrepancy between whole rock Hf and zircon in situ Hf isotopes. These inconsistences are understood to result from different mineral crystallization timings during mixing between endmembers with distinct isotopes. This highlights the need for detailed whole-rock or non-zircon phases Hf isotopic investigation in order to develop a comprehensive understanding of the granitoids of hybrid origin.