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

摘要

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 ~(40)Ar/~(39)Ar 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 ~(40)Ar/~(39)Ar MDD modeling result combined with the zircon U-Pb age reveals a rapid average cooling history (11.5 ℃/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 ~(87)Sr/~(86)Sr 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 ~(87)Sr/~(S6)KSr: 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.