Early-Cretaceous Syenites and Granites in the Northeastern Tengchong Block, SW China: Petrogenesis and Tectonic Implications

Whole-rock major and trace element and Sr-Nd isotopic data, together with zircon LA ICP- MS in-situ U-Pb and Hf isotopic data of the syenites and granites in the Tengchong Block are reported in order to understand their petrogenesis and tectonic implications. Zircon U-Pb data gives the emplacement ages of ca. 115.3±0.9 Ma for syenites and 115.7±0.8 Ma for granites, respectively. The syenites are characterized by low SiO2 content （62.01-63.03 wt%） and notably high Na20 content （7.04- 7.24 wt%） and Na20/K20 ratios （2.02-2.10）, low MgO, Fe2OaT and TiO2, enrichment of LILEs（large-ion lithophile element） such as Rb, Th, U, K, and Pb） and obvious depletion HFSE（high field strength element; e.g. Nb, Ta, P, and Ti） with clearly negative Eu anomalies （dEu=0.53-0.56）. They also display significant negative whole-rock eNd（t） values of-6.8 and zircon eHf（t） values（-9.11 to -0.27, but one is ＋5.30） and high initial S7Sr/86Sr=0.713013. Based on the data obtained in this study, we suggest that the ca. l15.3Ma syenites were possibly derived from a sodium-rich continental crustal source, and the fractionation of some ferro-magnesian mineral and plagioclase might occur during the evolution of magma. The granites have high SiO2 content （71.35-74.47 wt%）, metaluminous to peraluminous, low Rb/Ba, Rb/Sr, and AI2Oa/（MgO＋FeOT＋TiO2） ratios and moderate （AI2Oa＋MgO＋FeOT＋TiO2） content. They show low initial a7Sr/86Sr （0.703408 to 0.704241） and eNd（t） values （-3.8 to -3.5）, plotted into the evolutionary trend between basalts and lower crust. Hence, we suggest that the granites were derived from the melting of mixing sources in the ancient continental crust involving some metabasaltic materials and predominated metasedimentary greywackes. Together with data in the literatures, we infer that the Early Cretaceous magmatism in the Tengchong block was dominated by magmas generated by the partial melting of ancient crustal material, which represent the products that associated to the closure of Bangong-Nujiang Meso-Tethys.

Source and Significance of Detrital Zircons from Mesozoic Sandstones of the Upper Yangtze Block, China

In this study, we report U–Pb and Lu–Hf isotopic data for zircons from the Mesozoic sandstones of the Upper Yangtze area, which provide critical constraints on the provenance of these sediments and further shed light on the crustal evolution of the Upper Yangtze block. The results of isotopic chronology indicate the following：（1） The provenances of the study area are very complex, and the tectonic evolution process is relatively closed.（2） The provenances are mainly Archean–Proterozoic crystalline basement or recycled material; Paleoproterozoic crustal accretion in the western margin of the Yangtze block and Neoproterozoic magmatic activities related to subduction of the western margin of the Yangtze block; early Cambrian oceanic magmatic activity, which resulted from the intraplate extension of the northern margin of the Yangtze block; late Ordovician–early Silurian magmatic activity in the northern Yangtze block and Hercynian–Indochina uplift and erosion during the Hercynian movement.（3） The Yangtze crustal growth is episodic, and an increasing amount of ancient recycled material became part of the magmatic activity, as the zircon U–Pb ages are relatively young.

GEOCHEMICAL CHARACTERISTICS OF THE EARLY MESOZOIC GRANODIORITES AND THEIR TECTONIC IMPLICATIONS

The early Mesozoic granodiorites (ca.165 Ma) in the northeastern Hunan Province (NEH) have SiO2=65.4-69.65%, K2O/Na2O=0.95-1.38 and K2O+Na2O>6%, A/CNK=0.96-1.13 and belong to metaluminous high-K calc-alkaline series. They are characterized by LREE and LILEs enrichment, and HFSE depletion with slightly negative Eu anomalies (Eu/Eu*=0.62-0.90). The initial 87Sr/86Sr ratios are in range from 0.711458 to 0.717461, and εNd values vary from -9.4 to -12.3, distinct from those of the contemporaneous granodiorites mantle-derived from the Southeastern Hunan Province (SEH) (87Sr/86Sr(i)=0.707962～0.710396, εNd(t)=-6.98～-2.30). By contrast, such signatures are roughly similar to those of the neighboring other Mesozoic granitic plutons (Eu/Eu*=0.30-0.70; 87Sr/86Sr >0.710; εNd = -12 to -16) in South China Block (SCB), which have been interpreted as the remelting products of Precambrian basement. The Proterozoic lower-middle crust is an important contributor to the petrogenesis of these early Mesozoic granodiorites in the NEH. An intracontinental extension setting is present in the northeastern Hunan Province at that time due to the demand of enough thermal transfer.