The growth and evolution of continental crust contributed by multiple sources in the East Kunlun Orogen during Early Paleozoic

Abstract

The spatiotemporal evolution of magmatism is pivotal to understand the growth mechanism and evolution of continental crust during major orogenic cycles. In this study, we investigate the extensive Early Paleozoic magmatic rock suites (487-390 Ma) from the East Kunlun Orogen (EKO), and attempt to gain insights into the growth and reworking of continental crust. The geodynamic setting of the EKO in the Early Paleozoic can be divided into early compressional setting during oceanic subduction and continental collision (stage 1: 487–427 Ma) and a subsequent extensional setting associated with slab break-off and post-collisional collapse (stage 2: 426–390 Ma). In stage 1, regional magmatism was dominated by I-type granitic rocks and lithosphere mantle-derived mafic rocks. The I-type granitic rocks in this stage incorporated materials from coeval mafic rocks, which were derived from partial melting of a continuously enriched mantle wedge resulted from interaction with oceanic slab. Modeling of Hf isotope reveals that at least 65% of juvenile materials were introduced to the continental crust by magmatism in this stage. In addition, the slab fluids were dominated by interaction with the mantle wedge during 487–450 Ma (stage 1–1) while the slab melts started to contribute to regional magmatism during 450–427 Ma (stage 1–2) with or without interaction with the mantle wedge, and the crustal growth rate significantly increased from stage 1–1 to stage 1–2. During the extensional setting in stage 2, large-scale mafic magma with a source of asthenosphere mantle underplated /intruded into the lower continental crust, providing heat for the crustal reworking, and producing the juvenile mafic lower continental crust. The granitic rocks in this period were dominated by S- and A-types with an age range of 426–418 Ma (stage 2–1), I-type granites at 417–408 Ma (stage 2–2), and A-type granites during 406–390 Ma (stage 2–3). The S-type and A-type granites in these periods were derived from the reworking of crustal sediments and calc-alkaline granitoids, respectively, whereas the I-type granites in stage 2–2 were the products of the reworking of mafic lower continental crust. The factors resulting in the generation of different granitic rocks in these substages are attributed to the slab break-off at stage 2–1, thickened lithosphere blocked the heat input from the asthenosphere to felsic crust at stage 2–2, and the lithosphere delamination at stage 2–3. Crustal maturity was enhanced during these periods through reworking, bringing more silicic units with higher incompatible elements to the shallower crust. Thus, magmatism sourced from lithosphere mantle and oceanic slab contributed to the continental crust growth in the early compressional setting, whereas crustal reworking was the key to enhance the crustal maturity in the later extensional setting.