In this paper the authors briefly introduce and review the new progress of the newly dis- covered Proterozoic Miaowan (庙湾) ophiolite and Neoproterozoic Huangling (黄陵) granitoids in the southern Huangling anticline in the Yangtze craton, and the tectonic evolution significance of assemblage and breakup of Rodinia and Columbia supercontinent in South China.
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 ℃ in average), 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, indicating that there was a significant granulite facies tectonothermal event in the northern Huangling anticline in the Paleoproterozoic, which may be related with tectonic thermal events of the metamorphism caused by the assembly of the Columbia supercontinent with South China. Moreover, the REE pattern is characterized by depletion in LREE while relatively flat in HREE, La N /Yb N =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.
The olivine-gabbroic rocks located at the Huangling 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 Fe O(6.60 wt.%–8.23 wt.%) but high Ca O(20.23 wt.%–21.25 wt.%) contents, however, plagioclase has high Al_2O_3(31.78 wt.%–32.37 wt.%), Ca O(16.08 wt.%–16.25 wt.%) and An(79–80) values, but low Na2 O contents(1.95 wt.%–2.11 wt.%). Spinel are magnesioferrite with characteristics of high contents of Mg O(13.65 wt.%– 13.68 wt.%), Fe O(23.27 wt.%–23.40 wt.%) and Al_2O_3(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 εHf 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.
Xingfu JiangSongbai PengTimothy M.KuskyLu WangHao Deng
黄陵穹窿位于华南扬子克拉通核部地区,出露有华南前南华纪最古老、面积最大的太古宙-古元古代崆岭杂岩（崆岭岩群）（高山和张本仁,1990;马大铨等,1997;Gao et al.,1999,2011;Qiu et al.,2000;Zhang et al.,2006;Zheng et al.,2006;Jiao et al.,2009;Guo et al.,2014）,受后期新元古代黄陵花岗杂岩体侵入影响,大体以雾渡河大断裂为界分隔为南、北部两部分（也称为南、北崆岭群）,是扬子克拉通前南华纪基底最具代表性的岩石记录,一直受国内外地质学界的高度关注,
The Miaowan (庙湾) ophiolite is a highly dismembered ophiolitic complex cropping out near the northern margin of the Yangtze craton. The rocks of this complex consist of, from bottom to top, harzburgite tectonite locally containing podiform chromite, dunite, layered and isotropic gabbro, a sheeted dike complex (SDC), meta-pillow lavas with chert pods and layers, and tectonically intercalated marble. The SDC is a very important and significant part of the Miaowan ophiolitic sequence, and grades downward into gabbro and ultramafic rocks, and upward into meta-pillow lavas. Some dikes preserve one-way chilled margins, typical of extensional ophiolitic settings, whereas most preserve double chilled margins, in cases where the chilling direction can be determined. The SDC is mainly com-posed of meta-diabase (dolerite), meta-plagiogranite , and small amounts of meta-gabbro and ultramafic rocks. LA-ICP-MS zircon dating yields an upper intercept age of 1 026±79 Ma for one meta-plagiogranite, 1 043±23 Ma for a second meta-plagiogranite and 1 096±32 Ma for one meta-gabbro at the bottom of the SDC, suggesting formation of the SDC at circa 1 026-1 096 Ma, consistent with the recently determined formation age of the Miaowan ophiolite. Sparse geochemical data on the meta-diabase indicate that the protolith was a sub-alkaline, low-potassium tholeiite similar to mid-ocean ridge basalt (MORB). The chondrite-normalized rare earth element (REE) patterns of the meta-diabase are generally flat ((La/Yb) N =0.56-0.94), with a slight depletion in LREE, but no obvious Eu anomalies. Given that the meta-plagiogranites show evidence of formation in a suprasubduction zone environment, we suggest that the basalts were originally island arc tholeiites, perhaps formed in an extensional forearc setting. The geochemistry of the meta-diabase and plagiogranite from the sheeted dikes, together with regional relationships, all agree with the previous interpretations that the Miaowan ophiolite formed in a suprasubduction zone setting.
The epidosites are interpreted to form in upflow zones at the base of ore-forming oceanic hydrothermal systems that vent as black smokers on the sea floor. This study presents new field, major and trace element, and oxygen isotope data for the recently discovered epidosites in the ca. 1.0 Ga Miaowan (庙湾) ophiolite located near the northern margin of the Yangtze craton. The epidosites occur mainly in the cores of strongly deformed, lensoidal amphibolites. Field observations, major and trace elements and oxygen isotopes suggest that the epidosites were formed by metasomatism of ocean floor basalts, diabase dykes, and gabbros during seafloor hydrothermal alteration.
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 deformation. 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 partial 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
