Post-collisional, potassic monzonite–minette complex (Shahewan) in the Qinling Mountains (central China): 40Ar/ 39Ar thermochronology, petrogenesis, and implications for the dynamic setting of the Qinling orogen

Abstract

The nondeformed, postcollisional Shahewan monzonite–minette complex in the North Qinling orogenic belt, central China, is composed of potassic monzonites and mafic minette dykes. The monzonites and minette dykes were coevally intruded as evidenced by a zircon U–Pb age of 211 ± 2 Ma for the monzonites and a 40Ar/ 39Ar age of 209.0 ± 1.4 Ma on the biotite from the minette dykes. These dates also indicate that the Shahewan complex postdated shortly the cessation of convergent deformation in the Qinling orogenic belt. The K-feldspar 40Ar/ 39Ar MDD modeling result combined with the zircon U–Pb age reveals a rapid average cooling history (11.5 °C/Ma) for the complex from 211 to 150 Ma, implying a fast exhumation within a short period of time. The minettes take features of ultrapotassic rocks. Their geochemical characteristics, such as high MgO (Mg# up to 80), low TiO 2 (0.38–0.62 wt.%) and FeO (2.52–3.42 wt.%), enrichment in LILEs and LREEs, and depletion in HFSE, together with primitive initial 87Sr/ 86Sr ratios (0.70444–0.70562) and slightly negative ε Nd( t) (−3.4 to −2.5), suggest that they were derived from an enriched refractory lithospheric mantle. The monzonites have the similar geochemical (incompatible element enrichment, Nb, Ta, Ti depletion) and isotopic (initial 87Sr/ 86Sr: 0.70513–0.70646; ε Nd( t): −0.5 to −3.6) imprints, indicating they were derived from a common source. The dynamic setting in which the Shahewan complex formed is discussed. The Shahewan complex formed most likely in a postcollisional extensional setting. Its formation requires a high enough temperature to partially melt an enriched, refractory mantle source. Considerable uplift is necessitated for the rapid exhumation. These may be explained by convective mantle thinning or slab-breakoff mechanisms. The enrichment features in the lithospheric mantle source can be interpreted by injection of small percentage (<1%) partial melts from the convective mantle, or by addition of the metamorphic fluids from the subducted South China lithosphere.