Determination of the Neoproterozoic Shicaogou Syn-collisional Granite in the Eastern Qinling Mountains and Its Geological Implications

The Shicaogou granite has been identified as a magnesian (Fe-number=0.71-0.76), calcic to calc-alkalic (MALI=3.84-5.76) and peraluminous (ASI=1.06-1.13) granite of the syn-collisional S-type, with high SiO2(>71%), A12O3 (>13%) and Na2O+K2O (6.28%-7.33%, equal for NaO2 and K2O). Trace element and REE analyses show that the granite is rich in LILE such as of Rb, Sr, Ba and Th, and poor in HFSE like Yb, Y, Zr and Hf. Its Rb/Sr ratio is greater than 1; the contents of Nb and Ta, and the ratio of Nb/Ta as well as the REE geochemical features (e.g. REE abundance, visible fractionation of LREE and HREE and medium to pronounced negative Eu anomalies) are all similar to those of crust-origin, continent-continent syn-collisional granite. Moreover, the granite exhibits almost the same pattern as that of the typical continent-continent syn-collisional granite on the spider diagram and all samples fall within the syn-collisional granite field.The cathodoluminescence (CL) investigations have revealed that the zircon from the Shicaogou granite represents a typical magmatic product characterized by its colorless, transparent and euhedral crystals, and distinct zoning of oscillatory bands. Residual cores of irregular zircon can be found in a few enhedral grains. Trace element studies of the zircon grains, with high contents of P, Y, Hf, Th, U and REE and high ratios of Th/U, obviously positive Ce anomalies and HREE enrichment compared to LREE, also result in the same conclusion.The LA-ICP-MS U-Pb isotopic data from 24 spots of 21 zircon grains demonstrate that 20 spots in the oscillatory zone yield an average weighted 206Pb/238U age of 925±11 Ma, indicating that the Shicaogou granite was formed in the Neoproterozoic. Combined with other Neoproterozoic syn-collisional granites found in the study area, the present geochronological determination can further reveal that collision-amalgamation events could have occurred among some continental blocks in the Qinling orogenic belt during the Neoproterozoic. This in turn provides an accurate chronological constraint on the Neoproterozoic break-up and convergence in the belt.

U-Pb Zircon Geochronology and Geochemistry of Granitoids in the Douling Group in the Eastern Qinling

LA-ICPMS U-Pb zircon dating of the Sanpinggou, Gangou and Fengzishan granitoids in the Douling Group of the Eastern Qinling yields ages of 760-685 Ma, which represents a strong tectono-magmatic event in the southern Qinling during the late Neoproterozoic. Geochemical data show that these intrusions have wide compositions ranging from minor gabbros through diorites to granodiorites. They are relatively enriched in LILE, poor in HFSE and strongly depleted in Nb and Ta, displaying affinities of Ⅰ-type granites formed in an active continental margin with oceanic subduction. In contrast to granitoids, gabbros and enclaves in the granitoids have higher REE abundances, relatively flat REE patterns, lower LILE, slightly higher HFSE and more depletion in Nb and Ta. All these suggest that the gabbros were formed by partial melting of the upper mantle above the subduction zone and the granitoids by the partial melting of the lower crust. Combined with regional geological data, the subduction-related granitoids in the Douling Group, together with the Tuwushan A-type granite with an age of 725 Ma and contemporaneous basic dikes in the Wudang Block, provide evidence for local subduction of oceanic basins between different blocks during the rifting in the Southern Qinling in the Neoproterozoic. Thus, the coexistences of various magmatic rocks formed in different tectonic environments indicate a complicated tectonic evolution and variety of tectonic frameworks in the Qinling area in the Neoproterozoic.

Zircon U-Pb Ages of Bottom and Top Parts of the Zhangjiakou Formation in Lingyuan Area(West Liaoning Province) and Luanping Area (North Hebei Province) and Their Significance

In Lingyuan region of West Liaoning Province, the Zhangjiakou Formation (J_3z) and the Yixian Formation (K_1y) display an angular unconformity. That is, the Lower Mesozoic strata of the Zhangjiakou Formation are ENE (near E-W) oriented, while the overlying strata of the Yixian Formation exhibit an NNE orientation. The results of LA-ICP-MS zircon U-Pb ages show the Zhangjiakou Formation formed from about 130 Ma to 132 Ma in Lingyuan and 135 Ma to 136 Ma in Luanping (North Hebei Province), respectively. Three conclusions can be drawn: (1) The Zhangjiakou Formation in Lingyuan is comparable to that in Luanping, with the volcanic rocks of the Zhangjiakou Formation from Lingyuan being younger than those from Luanping. (2) 5-6 Ma difference between the top of the Zhangjiakou Formation and the bottom of the Yixian Formation in Lingyuan proves the angular unconformity between the two formations; and it reflects that the 5-6 Ma interval period is the main period of the transition of tectonic framework in Mesozoic in North Hebei and West Liaoning. In the interval period, the magmatic action went up to high tide in Mesozoic in the northeast of China. Moreover, after the interval period, the “Rehe fauna” developed into “erupted” period. This reflects that the interval period is also an important biological interface in Northern Hebei and Western Liaoning. (3) The Dabeigou Formation in Luanping should correspond to the upper part of the Zhangjiakou Formation, but not to the lower part of the Yixian Formation.

Zircon U-Pb SHRIMP ages of high-pressure granulite in Yushugou ophiolitic terrane in southern Tianshan and their tectonic implications

Zircons from two high-pressure granulite samples from the Yushugou ophiolitic terrane, southern Tianshan have been investigated by cathodoluminescence, LAICPMS and ion microprobe (SHRIMP) for their internal textures, trace elemental compositions and in situ dating. Theweighted mean ages of these two samples are 392±7 and 390±11 Ma, respectively, representing the granulite-facies metamorphic age of the ophiolitic terrane, and indicating that the southern Tianshan ocean initiated its northward subduction since the early Devonian.

Zircon U-Pb ages of olivine pyroxenite xenolith from Hannuoba: Links between the 97—158 Ma basaltic underplating and granulite-facies metamorphism

U-Pb zircon dating by LA-ICP-MS andSHRIMP for one olivine pyroxenite yields complex agepopulations including Mesozoic ages of 97-158 Ma and 228 ±8.7 Ma, Early Paleozoic ages of 418—427 Ma, Paleoprotero-zoic age of 1844±13 Ma, Neoarchean age of 2541 ± 54 Ma andmiddle Archean age of 3123 ± 4.4 Ma. The 97—158 Ma and228 ± 8.7 Ma zircons show typical igneous oscillatory zona-tion in CL images, suggesting two episodes of magmaticevents. Overlapping of the 97-158 Ma ages with that ofgranulite xenoliths indicates that the Mesozoic granu-lite-facies metamorphism was induced by heating from thebasaltic underplating at the base of the lower crust. Bothprocesses lasted at least from about 158 to 97 Ma. Ages of 418--427 Ma could be records of the subduction of Mongoliaoceanic crust under the North China craton. Ages of 1.84 Ga, 2.54 Ga and 3.12 Ga correspond to the three importantcrust-mantle evolutionary events in the North China craton,and imply preservation of Precambrian lower crust in thepresent-day lower crust.