Search

Abstract The Laoshan granite is an example of the widespread intraplate granitoids of early Cretaceous age in eastern continental China. The petrogenesis of these granitoids remains in dispute and we choose the Laoshan granite as the representative case to study these granitoids. Zircons from the Laoshan granite give a crystallization age of ~120 Ma, which is consistent with the emplacement age of ~ 126 Ma given by the bulk-rock Rb-Sr isochron. Representative samples from the granite show a large range of major element compositional variation (e.g., SiO 2 /MgO = 64 to 1937), reflecting a varying degree of fractional crystallization of plagioclase, alkali feldspar, amphibole, biotite and accessory phases as observed. The samples are enriched in light rare earth elements, Rb, Th and U, but depleted in Ba and Sr with negative Eu anomalies. The high 87Sr/86Sr (0.7083 to 1.2265) is largely caused by variably high Rb/Sr (~0.31 to 91 with an average of ~13) due to feldspar fractionation. The low ƐNd (t) (−13.8 to −19.5), ε Hf (t) (−14.6 to −24.4), and (206Pb/204Pb) i (16.244 to 17.304) are consistent with significant contributions of the lower continental crust to magmas parental to the Laoshan granite. The origin of the parental magmas is best understood as resulting from anatexis of the lower crust (~20% − 25% partial melting of the mafic granulite) triggered by and mixed with the underplating and intruding basaltic magmas. The basaltic magmas were likely derived from melting of the thinning lithosphere being transformed into the asthenosphere as the result of "basal hydration weakening" with the water ultimately coming from dehydration of the stagnant paleo-Pacific slab in the mantle transition zone. Highlights • The Laoshan granite resulted from fractional crystallization of the parental magma. • Anatexis of the LCC heated by and mixed with basic magma formed the parental magma. • The basal hydration weakening caused the lithosphere thinning with basic magmatism. [ABSTRACT FROM AUTHOR]

Abstract Jurassic-Cretaceous granitoids are widespread in eastern continental China and have been considered by many as resulting from paleo-Pacific subduction. However, the actual mechanism of their petrogenesis remains speculative. In order to address this important problem and on the basis of our regional study, we hypothesized that the coastal granitoids may result directly from the paleo-Pacific plate subduction, whereas the widespread granitoids in the continental interiors ultimately result from dehydration of the paleo-Pacific slab stagnated in the mantle transition zone (Niu et al., 2015). Here, we present the very first study testing this hypothesis. We sampled 18 Jurassic-Cretaceous granitoid plutons along a ~ 1300 km long traverse parallel to the inferred paleo-Pacific subduction from the southeast coastline to the Xiaoqinling in the continental interiors and carried out a detailed study on these plutonic samples, including zircon U Pb geochronology, bulk-rock major and trace element compositions and Sr-Nd-Pb-Hf isotopic characteristics. These plutons give varying zircon crystallization ages of ~146 to 100 Ma. They are mostly granitic and minor granodioritic, quartz monzonitic and syenitic in composition, enriched in large ion lithophile elements (LILEs), depleted in high field-strength elements (HFSEs) and have varying negative Sr and Eu anomalies. The plutons in the continental interiors show significant positive correlations of Nd (ε Nd (t) = −25.5 to −10.9) and Hf (ε Hf (t) = −31.5 to −11.3) isotopes with Pb isotopes (206Pb/204Pb (t) = 15.827 to 17.622), with the enriched endmember characterized by low ε Nd (t), ε Hf (t) and 206Pb/204Pb (t). The plutons towards the coastal region have relatively high ε Nd (t) (−9.0 to −5.2), ε Hf (t) (−11.2 to −4.1) and 206Pb/204Pb (t) (18.051 to 18.349). The coastal granitoids are best explained as resulting directly from subduction slab dehydration induced mantle wedge melting and resultant crustal anatexis, whereas the interior granitoids are best interpreted as resulting from mature crustal anataxis caused by basaltic magmatism associated with mantle lithosphere thinning, ultimately triggered by dehydration of paleo-Pacific slab stagnant in the mantle transition zone. Highlights • The interior granitoids have different sources and petrogenesis than coastal granitoids. • The interior granitoids result from basal hydration weakening induced crustal melting. • The coastal granitoids result directly from the paleo-Pacific plate subduction. [ABSTRACT FROM AUTHOR]

We present a detailed study of geochronology, mineral chemistries, bulk-rock major and trace element abundances, and Sr–Nd–Hf isotope compositions of the granodiorite and associated dioritic dikes in the Hongqilafu pluton at the northwestern margin of the Tibetan Plateau. The granodiorite and dioritic dikes yielded zircon U–Pb ages of ~ 104 Ma and ~ 100 Ma, respectively. The dioritic dikes comprise varying lithologies of gabbroic diorite, diorite porphyry and granodiorite porphyry, exhibiting a compositional spectrum from intermediate to felsic rocks. Their mineral compositions display disequilibrium features such as large major element compositional variations of plagioclase, clinopyroxene and amphibole crystals. These dioritic dikes are enriched in incompatible elements (Ba, Rb, Th, U, K) and Sr–Nd–Hf isotopes ( 87 Sr/ 86 Sr i : 0.7066 to 0.7071, ε Nd (t): − 5.3 to − 7.4, ε Hf (t): − 3.6 to − 6.2). We suggest that the dioritic dikes were most likely derived from partial melting of mantle wedge metasomatized by the subducted/subducting seafloor with a sediment component, followed by AFC processes with fractional crystallization of clinopyroxene, amphibole and plagioclase and assimilation of lower continental crust. The mantle-wedge derived magma parental to the dioritic dikes underplated and induced the lower continental crust to melt, forming the felsic crustal magma parental to the granodiorite with mantle melt signatures and having more enriched isotope compositions ( 87 Sr/ 86 Sr i : 0.7087 to 0.7125, ε Nd (t): − 9.5 to − 11.6, ε Hf (t): − 10.3 to − 14.1) than those of the dioritic dikes. The Hongqilafu pluton is thus the product of mantle–crust interaction at an active continental margin subduction setting over the period of several million years. This understanding further indicates that the closure timing of the Shyok back-arc basin and the collision between the Kohistan–Ladakh Arc and the Karakoram Terrane may have taken place later than ~ 100 Ma. [ABSTRACT FROM AUTHOR]