Structure, geochronology, and petrogenesis of the Late Triassic Puziba granitoid dikes in the Mianlue suture zone, Qinling orogen, China

摘要

We present structural field observations, in situ zircon U-Pb ages, Hf isotopic compositions, geochemistry, and Sr-Nd isotope data from the Puziba granitoid intrusions in the Qinling orogenic belt (China). These intrusions occur as ENE-WSW-oriented dike swarms in a highly deformed phyllite-matrix melange along several major fault zones within the Mianlue suture zone. Granitoid rocks have a relatively wide range of SiO2 (69-77 wt%, volatile-free) and total alkali contents (Na2O + K2O = 2.76-4.83 wt%), and K2O/Na2O ratios >1, and they mostly fall in the granite-granodiorite fields in the total alkalis versus silica diagram. They display relative enrichments of light rare earth elements (LREEs) and relatively flat heavy rare earth elements patterns (HREEs) with weak negative Eu anomalies (delta Eu = 0.56-0.95) on chondrite-normalized rare earth element (REE) diagrams. They show enrichment in Th and Zr and depletion in high field strength elements (HFSEs; e.g., Nb, P, and Ti). They are enriched in large ion lithophile elements (LILE) and LREEs and have low zircon epsilon(Hf)(t) (-2.8 to 4.2) values, relatively high initial Sr-87/Sr-86 ratios (0.7107-0.7188), and variable Mg# (22-46). Their U-Pb zircon crystallization age is ca. 215 Ma, but they also show inherited zircon age clusters in the Paleoproterozoic, Mesoproterozoic, Neoproterozoic, and Paleozoic, with two prominent peaks at ca. 753 Ma and ca. 805 Ma. The high Rb and Ta elemental abundances, smooth chondrite-normalized REE patterns, and epsilon(Nd)(t) values (-5 to -3) of the Puziba dike rocks characterize them as K-rich, calc-alkaline granites. Their magmas were derived from partial melting of garnet-amphibole-bearing lower crust that was triggered by slab breakoff-induced asthenospheric upwelling in the aftermath of the oblique collision of the North and South China blocks during the Late Triassic. Emplacement of the ENE-WSW-oriented Puziba dike swarms was associated with a transtensional stress regime along/across the collisional Mianlue suture zone.