SHRIMP Zircon U-Pb Ages and Geochemical Characteristics of the Neoproterozoic Granitoids in the Huangling Anticline and Its Tectonic Setting

SHRIMP zircon U-Pb dating of the Neoproterozoic Maoping （茅坪） series （Sandouping （三斗坪） rock suite） granites exposed in the southern part of the Huangling （黄陵） anticline shows that the formation time of Sandouping biotite-hornblende tonalite intrusion, Jinpansi （金盘寺） hornblende-biotite tonalite intrusion, and Longtanping （龙潭坪） monzogranite are 863±9, 842±10, and 844±10 Ma, respectively. Their geochemical features include A/CNK=0.98-1.06, from metaluminous to weakly peraluminous, δ=1.37-1.53, Sm/Nd=0.17-0.24, and RbN/YbN=1.1-3.62. These indicate that the granite rocks are supersaturated SiO2 calc-alkaline granitoids. The characteristic of Sr-Nd isotopic composition is that the values of εNd（t） and εsr（t） are -12.4 to -11.0 and 20.2-32.2, respectively. It also suggests that the material source of the granite rocks mainly originated from the crust, and they formed in a volcanic arc tectonic environment. These facts suggest that the occurrence of Neoproterozoic granitoids in the southern part of the Huangling anticline should be related to an arc environment along an active continental margin caused by southward subduction of oceanic crust beneath the northern Yangtze craton, and the formation age is not later than 863 Ma.

Granulite Facies Metamorphic Age and Tectonic Implications of BIFs from the Kongling Group in the Northern Huangling Anticline

We report preliminary results of a geochemical study on banded iron formations （BIFs） in the Zhaojiayangpo （赵家阳坡） area from the Kongling （崆岭） Group in the northern Huangling （黄陵） anticline, on the northern margin of the Yangtze craton. The CL （cathodoluminescence） images of zircons mostly have sector zoning, fir-tree zoning and patched zoning, and a few show core-rim tex-tures with rims having patched zoning. The calculated formation temperatures using the Ti-in-zircon thermometer are 660-808 ℃ （714 ℃C in aver-age）, all indicating that the BIFs underwent granulite facies metamorphism. The age of zir-cons with granulite facies metamorphism is 1 990±14 Ma by LA-ICP-MS U-Pb dating, indi-cating that there was a significant granulite fa-cies tectonothermal event in the northern Huangling anticline in the Paleoproterozoic, which may be related with tectonic thermal events of the metamorphism caused by the as-sembly of the Columbia supercontinent with South China. Moreover, the REE pattern ischaracterized by depletion in LREE while relatively flat in HREE, LaN/YbN=0.26, with a positive Eu anomaly （Eu/Eu^＊---1.59）, which reveals its hydrothermal sedimentation origin and it may have formed in the environment of submarine exhalation.

Geological Features and Deformational Ages of the Basal Thrust Belt of the Miaowan Ophiolite in the Southern Huangling Anticline and Its Tectonic Implications

The stratigraphic, structural and metamorphic features of the basal thrust belt of the ca. 1.0 Ga Miaowan （庙湾） ophiolite in the southern Huangling （黄陵） anticline, show that it can be divided into three tectono-lithostratigraphic units from north to south： mélange/wildflysch rock units, flysch rock units, and sedimentary rock units of the autochthonous （in situ） stable continental margin. The three units underwent thrust-related deformation during emplacement of the Miaowan ophiolitic nappe, with kinematic indicators indicating movement from the NNE to SSW, with the metamorphic grade reaching greenschist-amphibolite facies. LA-ICP-MS U-Pb geochronology of zircons from granite pebbles in the basal thrust-related wildflysch yield ages of 859±26, 861＋12 and 871±16 Ma; whereas monzonitic granite clasts yield an age of 813±14 Ma. This indicates that the formation age of the basal thrust belt is not older than 813±14 Ma, and is earlier than the earliest formation time of the majority of the Neoproterozoic Huangling granitoid intrusive complex, which did not experience penetrative ductile deforma-tion. These results suggest that the northern margin of the Yangtze craton was involved in collisional tectonics that continued past 813 Ma.This may be related to the amalgamation of the Yangtze craton with the Rodinia supercontinent. Through comparative study of lithology, zircon geochronology, REE patterns between granodiorite and tonalite pebbles in the basal thrust-zone conglomerate, it can be concluded that the pebbles are the most similar to the Huanglingmiao （黄陵庙） rock-mass （unit）, implying that they may have come from Huanglingmiao rock-mass. Zircon cores yield xenocrystic ages of 2 074±120 Ma, suggesting that the protolith of the Neoproterozoic Huangling granitoid intrusive complex may have originated from par-tial melting of older basement rocks, that is to say there may be Paleoproterozoic crystalline basement in the southern Huangling anticline. The ages of xenocrystic zircons in the granite pebbles in the basal-thrust congiomerate/wildflysch show a correlation with the age spectra from Australia, implying that the terrain that collided with the northern margin of the Yangtze craton and emplaced the Miaowan ophiolite at ca. 813 Ma may have been derived from the Australian segment of Rodinia.

Petrogenesis and Geotectonic Significance of Early-Neoproterzoic Olivine-Gabbro within the Yangtze Craton: Constrains from the Mineral Composition, U-Pb Age and Hf Isotopes of Zircons

The olivine-gabbroic rocks located at the Huangiing anticline within the Yangtze Craton are dated at circa 857-854 Ma by LA-ICP-MS method. The rocks belong to the sub-alkaline series and consist of pyroxene （35%-40%）, plagioclase （40% -45%）, olivine （8%-10%） and spinel （3%-5%）. Olivine has Fo values of 73-83 that is classified as chrysolite. Pyroxene has relative low contents of FeO （6.60 wt.%-8.23 wt.%） but high CaO （20.23 wt.%-21.25 wt.%） contents, however, plagioclase has high A1203 （31.78 wt.%o-32.37 wt.%）, CaO （16.08 wt.%-16.25 wt.%） and An （79-80） values, but low Na20 contents （1.95 wt.%-2.11 wt.%）. Spinel are magnesioferrite with characteristics of high contents of MgO （13.65 wt.%- 13.68 wt.%）, FeO （23.27 wt.%-23.40 wt.%） and A1203 （62.43 wt.%-62.74 wt.%）. Chemical compositions of these minerals are similar to those of gabbro rocks that were formed in the post-orogeny environment. The olivine-gabbro samples have negative zircon eHf values （-16.57±0.47） that resemble the mafic rocks in the same region, indicating that they are derived from the extremely enriched mantle source. On the compilation of documented Neoproterozoic mafic rocks in the Yangtze Craton, it is proposed that the mantle in the northern Yangtze Craton has experienced different degrees enrichment during the Neoproterozoic.