Petrogenesis of Early Silurian granitoids in the Longshoushan area and their implications for the extensional environment of the North Qilian Orogenic Belt, China
Abstract
Analyses of the zircon U-Pb age, mineralogical chemistry and whole-rock geochemistry of the Jiling pluton along the southern margin of the Alxa Block were conducted. The results of these analyses will provide crucial information for investigations of Paleozoic crust-mantle interactions in the North Qilian Orogenic Belt (NQOB) and explorations of the transition age from a compressional to extensional environment during the collision between the Alxa Block and the Qilian-Qaidam Block. The zircon dating results reveal that the Jiling pluton is composed of Early Silurian granitoids (435-442 Ma) with contemporaneous mafic microgranular enclaves (MMEs). Most granitoids contain amphibole and titanite and have A/CNK values <1, illustrating that they belong to amphibolite-rich calc-alkaline granitoids (ACGs) and K-rich and K-feldspar porphyritic calc-alkaline granitoids (KCGs). Both the granitoid and MME samples display variable initial Sr and Nd isotopic compositions. We suggest that the Jiling pluton was formed by crust-mantle-derived magma mixing. The high HFSE content of the MMEs indicates that the mantle-derived magmas were derived from melting of the mantle wedge metasomatized by adakitic magma, and the crustal-derived magmas were derived from melting of Paleoproterozoic meta-basaltic rocks in the Longshoushan Group. The discovery of the Jiling granitoids indicates that the NQOB was dominated by a compressional environment during Middle Ordovician-Early Silurian in response to the collision between the Qilian-Qaidam Block and the Alxa Block, and the initial extensional environment began before 442 Ma.
- Publication:
-
Lithos
- Pub Date:
- October 2019
- DOI:
- 10.1016/j.lithos.2019.05.029
- Bibcode:
- 2019Litho.342..152W
- Keywords:
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- Extensional environment;
- Microgranular mafic enclave;
- Magma mixing;
- Early Silurian;
- North Qilian Orogenic Belt