Genesis of LCT Pegmatites during Early Paleozoic Orogeny of the North Qinling Orogenic Belt, China: Emplacement Conditions and Structural Control

Abstract

The Guanpo pegmatite field in the North Qinling orogenic belt (NQB), China, hosts the most abundant LCT pegmatites. However, their emplacement conditions and structural control remain unexplored. In this contribution, we investigated it combining pegmatite orientation measurement with oxygen isotope geothermometry and fluid inclusion study. The orientations of type A1 pegmatites (P_f < σ₂) are predominantly influenced by P- and T-fractures due to simple shearing in Shiziping dextral thrust shear zone during D₂ deformation, whereas type A2 pegmatites (contemporaneous with D₄) are governed by hydraulic fractures aligned with S₀ and S₀₊1 stemming from fluid pressure (P_f <σ₂). Additionally, type B pegmatites (P_f ≤ σ₂) exhibit orientations shaped by en echelon extensional fractures in local ductile shear zones (contemporaneous with D₃). The albite-quartz oxygen isotope geothermometry and microthermometric analysis of fluid inclusions in elbaites from the latest pegmatites (including types B and A2) suggest that the crystallization P-T for late magmatic and hydrothermal stages are 527.5–559.2°C, 320°C, 3.1–3.6 kbar and 2.0 kbar, respectively. Our observations along with previous studies suggest that the genesis of the LCT pegmatites was a long-term, multi-stage event during early Paleozoic orogeny (including the collision stage) of the NQB, and was facilitated by various local fractures.