Qinling Group Research Articles

Origin, Phylogeny, and Taxonomy of Lenoks of the Genus Brachymystax (Salmonidae): Available Data, Their Interpretation, and Unresolved Problems

The reproductive and phylogenetic relationships of lenoks of the genus Brachymystax are considered based on analysis of 30 allozyme loci and two fragments (411 and 987 base pairs) of the mitochondrial DNA control region. The presence of three phylogenetic lenok groups, the blunt-snouted and sharp-snouted groups from Russia and neighboring countries and the Qinling group from China and South Korea, has been confirmed. It is assumed that the center of origin of the genus Brachymystax was Primorye and the blunt-snouted lenok from this region is closest to the ancestral form. Modern assumptions on the taxonomic status of different forms of lenok are contradictory both in the number of species (from one to five) and in their composition. The identification of two or three species in the genus Brachymystax is most reasonable. The main problems that should be solved to clarify the phylogeny and taxonomy of representatives of this genus are indicated.

Petrogenesis and geodynamic settings of the Devonian adakites and A-type granites in the North Qinling orogen

The Devonian adakites and A-type granites developed in the eastern section of the North Qinling (NQ) orogen in Central China records the evolution of the East Qinling orogen. In this study, petrological, zircon UPb and LuHf isotopic, whole rock element and SrNd isotopic analyses were carried out to investigate the origin and geodynamic setting of the Sikeshu and Shenxianya plutons in NQ. The Sikeshu two-mica granites formed at 415.7 ± 2.3 Ma possess geochemical characteristics similar to those of adakites, while the Shenxianya monzogranites formed at 386.4 ± 1.8 Ma exhibit A-type granite characteristics. Both plutons show enriched SrNd isotopic characteristics and display enrichment and depletion of the zircon Hf isotope. Results show that the Sikeshu two-mica granites were derived by partial melting of a combined source from Erlangping mafic rocks and the Qinling Group, while the Shenxianya monzogranites were derived by partial melting of lower crust granulite. In the late Late Silurian, the mantle-derived magma produced by the slab failure underplated the thickened lower crust, forming the Sikeshu two-mica granites with adakitic features. The NQ underwent a transition from compression to extension during the late Late Silurian-Early Devonian following the closure of the Shandfan Ocean. The mantle-derived magma formed by delamination underplated the lower crustal granulites, generating the Shenxianya A-type granite.

Hf-Nd isotope decoupling in S-type granitoids caused by disequilibrium melting of metasedimentary rocks: Evidence from the North Qinling Terrane, Central China

The intrinsic assumption of radiogenic isotope tracing is that isotope systems maintain equilibrium between melt and source. However, this assumption is not naturally correct, particularly for S-type granites, which are formed by partial melting of metasedimentary rocks. The S-type metagranites from the North Qinling Terrane show various degrees of HfNd isotope decoupling and discrepancies between whole-rock and zircon Hf isotopes. Zircon UPb dating yields emplacement ages from 946 to 890 Ma. Their high δ18O values, (87Sr/86Sr)i ratios, and negative εNd(t) values overlapped with metasedimentary rocks in the Qinling Group imply that they are derived from partial melting of the Qinling Group. The low Rb/Ba, Rb/Sr, Al2O3/TiO2, and high CaO/Na2O ratios indicate that their sources are metagraywackes. The negative correlation between decoupling degrees of HfNd isotopes and Zr concentrations suggests that the incomplete dissolution of zircon during partial melting results in the decoupling. The discrepancy between whole-rock and zircon Hf isotopes can be attributed to the combined effect of incomplete dissolution of zircons and the release of various Hf isotope compositions from partly dissolved inherited zircons to local melts. The compiled literature data suggest that whole-rock HfNd isotope decoupling of most metagraywacke-derived S-type granites is caused by incomplete melting of zircons, whereas those of metapelite-derived S-type granites may be controlled by both incomplete melting of zircons and source inheritance.

Late Palaeozoic tectonic transformation of the North Qinling Orogenic Belt: Insight from the geochronology, geochemistry and Sr‐Nd‐Hf isotopes of Carboniferous magmatic rocks

The North Qinling Orogenic Belt (NQB) records the pivotal geological information for understanding the Palaeozoic evolution of the Qinling Orogenic Belt (QOB). Previous studies mainly focused on the subduction–collision process along the Shangdan suture zone before the Early Devonian. However, as the significant interim period between the Early Palaeozoic and Mesozoic orogeny, the Late Palaeozoic tectonic evolution of the NQB remains poorly understood. Fortunately, the Carboniferous magmatic rocks discovered this time provide substantial geological evidence for revealing the Late Palaeozoic tectonic history of the Qinling Orogenic Belt. This paper will provide a detailed analysis of zircon U–Pb ages, geochemical characteristics and Sr‐Nd‐Hf isotopic compositions of Carboniferous rocks. The Carboniferous magmatic rocks are categorized as highly differentiated S‐type granite and monzodiorite, formed at 350 ± 2.4 Ma and 353 ± 5.2 Ma, respectively. The granite shows weak peraluminous (A/CNK = 1.02–1.11) and shoshonite nature. Based on the isotopic composition and trace element characteristics, we propose that the Qinling Group paragneiss is the primary source of the Carboniferous granite. Monzodiorite is characterized by enriched LREEs and LILEs (Ba, K and Pb), depleted HFSEs (Th, Nb, Ta and Ti) and enriched Sr‐Nd isotopic composition. Monzodiorite magma source region consists of continental crustal material and lithospheric metasomatized mantle. Carboniferous magmatic rocks are the product of an extensional tectonic setting, which indicates the NQB tectonic regimes transition from compression to extension during the Carboniferous.

Petrogenesis and Tectonic Implications of the Neoproterozoic Peraluminous Granitic Rocks from the Tianshui Area, Western Margin of the North Qinling Terrane, China: Evidence from Whole-Rock Geochemistry and Zircon U–Pb–Hf–O Isotopes

The source and petrogenesis of peraluminous granitic rocks in orogenic belts can provide insights into the evolution, architecture, and composition of continental crust. Neoproterozoic peraluminous granitic rocks are sporadically exposed in the Tianshui area of the western margin of the North Qinling Terrane (NQT), China. However, the source, petrogenesis, and tectonic setting of these rocks still remain unclear, which limits our understanding of the Precambrian tectonic and crustal evolution of the Qinling Orogenic Belt (QOB). Here, we determined the whole-rock geochemical compositions and in situ zircon U–Pb ages, trace-element contents, and Hf–O isotopic compositions of a series of peraluminous granitic mylonites and granitic gneisses in the Tianshui area at the west end of North Qinling. Zircon U–Pb dating revealed that the protoliths of the studied granitic mylonites and granitic gneisses crystallized at 936–921 Ma. The granitic rocks displayed high A/CNK values (1.12–1.34) and were enriched in large-ion lithophile elements (e.g., Rb, Ba, Th, U, and K) and light rare earth elements, and they were depleted of high-field-strength elements (e.g., Nb, Ta, and Ti). These rocks showed variable zircon εHf(t) (−12.2 / 9.7) and δ18O (3.56‰ / 11.07‰) values, suggesting that they were derived from heterogeneous crustal sources comprising predominantly supracrustal sedimentary rocks and subordinate igneous rocks. In addition, the U–Pb–Hf isotopic compositions from the core domains of inherited zircons were similar to those of detrital zircons from the Qinling Group, suggesting that the Qinling Group was an important crustal source for the granitic rocks. The lithological and geochemical features of these granitic rocks indicate that they were generated by biotite dehydration melting of heterogeneous sources at lower crustal depths. Combining our results with those of previous studies, we suggest that the NQT underwent a tectonic transition from syn-collision to post-collision at 936–874 Ma in response to the assembly and breakup of the Rodinia supercontinent.

A missing piece between Laurentia and the North China Craton in Rodinia: Evidence from metasedimentary rocks of the North Qinling Terrane in central China

The North Qinling Terrane was a microcontinent in the Late Neoproterozoic and then subducted northwards beneath the North China Craton for regional metamorphism in the Early Paleozoic. In order to constrain its location in supercontinent Rodinia, metasedimentary rocks were sampled from it for a combined study of whole-rock major and trace elements as well as zircon U-Pb ages and Lu-Hf isotopes. The metasedimentary rocks show greywacke compositions with A/CNK ratios of 0.99–1.28 and upper continental crust-like trace element patterns. They have metamorphic ages of ca. 510–490 Ma, recording continental collision for the regional metamorphism. The youngest detrital zircon U-Pb ages from three metasedimentary rocks are 1161 ± 54 Ma, 954 ± 15 Ma, 861 ± 10 Ma, respectively. In combination with ages of granitic intrusions of 960 to 890 Ma in the target region, their deposition ages are constrained to be 1110–960 Ma, ca. 960 Ma and ca. 860 Ma, respectively. The cumulative proportions of the difference between the detrital zircon crystallization ages and the deposition ages for the sediments of ca. 960 Ma and ca. 860 Ma suggest their deposition in a collisional setting. Detrital zircon age spectrum from the Qinling Group yields two major populations at 1000–900 Ma and 1700–1400 Ma and three subordinate populations at 900–800 Ma, 1300–1200 Ma and 1800–1700 Ma with rare Neoarchean grains. Neoproterozoic detrital zircons mainly have negative εHf(t) values, consistent with Neoproterozoic granitoids in the North Qinling Terrane. On the other hand, Mesoproterozoic detrital zircons mainly have positive εHf(t) values, similar to those in East Laurentia and East Greenland. This means that the major provenance of sediments in the Qinling Group was the North Qinling Terrane itself and East Laurentia, supporting the connection between the North Qinling Terrane and East Laurentia in Rodinia prior to its breakup. In combination with the coeval strata in the southern margin of the North China Craton, we propose that the North Qinling Terrane is a missing piece connecting the southern margin of the North China Craton with the northern part of East Laurentia in Rodinia in the early Neoproterozoic.

SIMS U-Pb Dating of Uraninite from the Guangshigou Uranium Deposit: Constraints on the Paleozoic Pegmatite-Type Uranium Mineralization in North Qinling Orogen, China

Pegmatite-type uranium mineralization occurs in the Shangdan domain of the North Qinling Orogenic Belt, representing a significant uraniferous province. The Guangshigou deposit is the largest U deposit of the district. Within the North Qinling area, a series of Caledonian granitic igneous rocks intruded the Proterozoic metamorphic rocks of the Qinling Group in two magmatic stages: (i) the Early Silurian Huichizi granite that was derived from a low degree of partial melting of thickened lower basaltic crust combined with mantle-derived materials following the subduction of the Shangdan Ocean; and (ii) the Late Silurian–Early Devonian Damaogou granite and associated pegmatites derived from the same source but emplaced in a late tectonic post-collisional extension environment. In the Guangshigou deposit, the U mineralization mainly occurs as uraninite disseminated in U-rich granitic biotite pegmatites, which formed by assimilation-fractional crystallization magmatic processes. Petrographic observations showed evidence for coeval crystallization of uraninite and other rock-forming minerals of the host pegmatite including quartz, feldspar, biotite, zircon, monazite, apatite, and xenotime. In addition, the low U/Th ratios (~19) and Th, REE, and Y enrichments characterized a magmatic origin for uraninite, which was likely derived from fractionated high-K calc-alkaline pegmatitic magma that experienced various degrees of crustal material contamination. In situ U-Pb isotopic dating performed by Secondary-Ion Mass Spectrometry (SIMS) on uraninite from the Guangshigou deposit yielded a crystallization age of 412 ± 3 Ma, which is concomitant (within errors) with the emplacement age of the host pegmatite (415 ± 2 Ma) and constrained the U ore genesis to the Early Devonian, which corresponds to the late Caledonian post-collisional extension in the North Qinling area. Uraninite then experienced various degrees of metamictization and/or post-Caledonian hydrothermal alteration characterized by an alteration rim associated with coffinite, chlorite and limonite. Finally, the characteristics of the pegmatite-related Guangshigou deposit exhibiting Th-rich uraninite which was the product of assimilation-fractional crystallization of pegmatitic magma defined a model significantly different than the one established for the world-class Rössing deposit characterized by Th-poor uraninite hosted in alaskite dykes formed by low degree of partial melting of U-rich metasediments.

Water-Flux Melting of Amphibolite in Paleozoic Leucogranite from the North Qinling, Central China: Implication for Post Collisional Setting

Paleozoic granites can provide important insights of crustal differentiation and collision process of the North Qinling terrane. The Dafanggou leucogranites that intruded in the Qinling Complex is a Paleozoic granite with zircon U-Pb age of 404±6.4 Ma (MSWD=0.13,n=9). The leucogranites display moderate SiO2 (68.4 wt.% to 71.7 wt.%) content and high Na2O/K2O (1.07 to 2.74) and Sr/Y (42 to 65) ratios,and low A/CNK (1.04 to 1.08) and Rb/Sr (0.09 to 0.16) ratios. Combined with negative eNd(t) (-10.4) and eHf(t) (-9.35 to -0.25) values and ancient TDM2 ages (1.2 to 1.7 Ga),suggesting the leucogranite are derived from ancient amphibolitic crust in the Qinling Group. In addition,the absence of coeval mafic igneous rocks or mafic enclaves occurred in the Dafanggou pluton and the low TZrn (~700℃) of leucogranites preclude that the leucogranites derived from high temperature dehydration melting of amphibolite. Thus,we propose that the occurrence of the Dafanggou leucogranite indicates that water-flux melting of middle-lower crust in post-collision setting may be a potential model for the genesis of granites in collisional orogenic belt.

Multistage Metamorphic Evolution of Retrograded Eclogites from the Songshugou Complex, Qinling Orogenic Belt, China

Abstract The Qinling Orogenic Belt is one of the major collisional orogens in eastern Asia and marks the boundary between the North China Craton and South China Craton. The Songshugou complex is the largest basic to ultrabasic body to be found in the North Qinling Belt, and was emplaced as a lens-shaped body at the southern margin of the Qinling Group. A detailed petrological investigation of garnet amphibolite, augen amphibolite and well-foliated amphibolite together with garnet zoning patterns of major and trace elements, inclusions in garnet, and thermodynamic modelling indicate a multistage metamorphic history. Garnets clearly show characteristics of discontinuous growth, as they display optically light-colored snowball-textured cores surrounded by a darker mantle with few inclusions as well as chemically a sudden increase in grossular and decrease in almandine components. A partly resorbed rim is not recognized optically but mineral inclusions and a discontinuous chemical composition of garnet are proof of this third garnet growth stage. Rare earth element distribution patterns of garnet also show clear evidence for discontinuous growth and allow us to identify the reactions responsible for garnet growth. Garnet core compositions as well as amphibole inclusions allow us to constrain a P–T window where this rock equilibrated in a first stage. Calculated pseudosections and the application of the garnet–amphibole thermometer indicate an upper amphibolite- to lower granulite-facies metamorphic episode at 630–740 °C and 0·7–0·9 GPa. The presence of relict omphacite as well as a discontinuously grown garnet mantle with rutile inclusions clearly places the peak metamorphic stage in the eclogite facies. Garnet (XGrs, XAlm, XPrp) and omphacite isopleths (XMg, XNa) constrain this event at 1·7–2·1 GPa and 570–650 °C. Consistent temperatures of 500–650 °C were also determined by clinopyroxene–garnet geothermobarometry for this event. Growth of an outermost rim as well as different stages of garnet breakdown to plagioclase + amphibole coronae and the nearly complete replacement of former omphacite by a variety of symplectites point to an intricate retrograde P–T path. In more strongly retrograded samples plagioclase + amphibole ± quartz pseudomorphs entirely replace former garnet grains. Certain coronae around garnets and symplectites also contain prehnite and pumpellyite, which formed during a late retrograde stage or during a different event at very low P–T conditions (250–350 °C). Based on the detailed petrological study, we favour a multistage metamorphic history of the Songshugou metabasic rocks. The age of the eclogite-facies metamorphic event must be related to the deep subduction of the Songshugou complex during the early Paleozoic, although the age of garnet core growth remains enigmatic. The development of garnet cores indicates an upper amphibolite-facies regional metamorphic overprint succeeded by an eclogite-facies event around 500 Ma and subsequent retrogression seen in replacement of garnet and formation of symplectite. The latest imprint evidenced by prehnite and pumpellyite may be the result of fluid infiltration during the fading orogenic phase or represents a low-temperature overprint by a later process, probably related to the uplift of the North Qinling terrane at around 420 Ma.

Metallogenic and tectonic implications of detrital zircon U–Pb, Hf isotopes, and detrital rutile geochemistry of late carboniferous karstic bauxite on the southern margin of the North China Craton

Late Carboniferous bauxite is widely developed on the North China Craton (NCC), and is referred to as the “G” layer bauxite. The bauxite ores of the southern NCC are closely related to the tectonic evolution of the NCC and its neighboring North Qinling Belt (NQB) to the south. In this study, based on the geological and mineralogical observations, we used the U–Pb and Hf isotopes of detrital zircons and chemical compositions of detrital rutiles from the Gunziying and Shibanhe bauxite deposits, along the southern margin of the NCC to study the metallogenic process of bauxite ores and their tectonic implications. One and two bauxite ore layers were observed in the Gunziying and Shibanhe bauixte deposits, respectively. The bauxite ore is dominated by diaspore, illite, anatase, kaolinite, pyrite, zircon, and rutile. Analyses of detrital zircons revealed that the largest group (∼432–515 Ma, at a peak age of ∼450 Ma) is spread throughout the Late Carboniferous bauxite deposits, with εHf(t) values of −5.7–21.4, similar to the zircons from the NQB. Similarly, the detrital zircons with peak ages of ∼950 Ma and age clusters at ∼1.1 Ga, ∼1.3 Ga, and ∼1.8 Ga as well as those ranging in age from 635 to 797 Ma, also exhibited εHf(t) values similar to the zircons from the NQB. In addition, the results suggested that 62 (73%) and 75 (88.2%) of detrital rutiles showed metapelitic protoliths and amphibolite/eclogite facies temperatures, respectively, while 23 (27%) and 10 (11.8%) exhibited metamafic protoliths and granulite facies temperatures, respectively. Based on the results above, we propose that the provenance of the bauxite ores on the southern margin of the NCC is primarily from the NQB. The dominant contributors were the Neoproterozoic Kuanping Group and the Meso–Neoproterozoic Qinling Group. The Archean-Proterozoic metamorphic rocks in the paleo-uplifts of the NCC were also likely involved in the bauxite formation. A probable outline of the bauxite-formation process on the southern NCC, former weathered residuals and subsequent deposition, is proposed; and a reasonable interpretation that the NQB was accreted to the NCC in the Late Carboniferous and formed a northward lying slope basin within the NCC, is supported.

Complex magma sources of late Mesozoic granites along the southern margin of the North China Craton: constraints from geochemistry and geochronology of the massive Heyu and Lantian plutons

ABSTRACT Mesozoic granitoid rocks in the eastern Qinling terrane along the southern margin of the North China Craton are significant for understanding geodynamics processes related to lithospheric delamination in the Late Mesozoic. In addition, these rocks have close genetic association with the largest Mo-(Au-Ag) polymetallic mineralization belt in China. In the present study, zircon U-Pb ages and elemental and Sr-Nd-Pb isotopic geochemistry are reported for two representative granite plutons (Heyu and Lantian) exposed in the eastern Qinling terrane. Granitoid rocks from both plutons are metaluminous to weakly peraluminous and are classified as I-type granite with formation ages of 145 ~ 135 Ma and 150 Ma, respectively. These rocks are characterized by adakite-like affinity without significant Eu anomalies. Both plutons have relatively high initial 87Sr/86Sr ratios ranging between 0.7063 and 0.7109 and variable εNd(t) values of – 29.2 to – 6.5. Their Pb isotopic compositions are comparable to those of old high-grade metamorphic basement rocks from the North China Craton but are different from those of country rocks from the Xiong’er and Taihua groups. These features indicate genesis via reworking of the lower crust beneath North China and partial involvement of juvenile crustal material to different extents. Occurrences of ca. 1000-Ma-old inherited zircon in the Lantian granites imply the contribution of the Qinling Group in North Qinling to the magma source(s). This magmatism occurred intensively in a transitional setting from compression to extension and was likely induced by lithosphere thinning and asthenosphere upwelling beneath eastern China during the Cretaceous.

Minor and trace element variations in chromite from the Songshugou dunites, North Qinling Orogen: Evidence for amphibolite-facies metamorphism

The Songshugou dunites are part of the ultramafic sequence of the Songshugou ophiolite within the Qinling Group metamorphic rocks, which are distributed in the northern part of the Shang–Dan Suture Zone (SDSZ) in central China. The dunites include lenses of coarse- and medium-grained dunite set in a fine-grained dunite host. Disseminated chromite within these dunites can be classified into three microstructural types: (1) partly altered chromite with primary cores surrounded by porous chromite rims; (2) porous chromite with abundant pores that are filled with chlorite; and (3) homogeneous chromite. The primary magmatic cores of the partly altered chromite (Chr) is depleted in Cr and enriched in Al and Mg. Porous chromite includes Cr-rich Chr and ferritchromite (Fe-Chr) that is enriched in Fe2+ but depleted in Al. Homogeneous chromite is mainly Cr-magnetite (Cr-Mag) that is enriched in Fe3+ and extremely depleted in Al. The primary Cr-poor chromite was gradually transformed to porous Cr-rich Chr and/or by Fe-Chr during metamorphism. Reaction of Cr-poor Chr with olivine (Ol) produces chlorite (Chl) and secondary Chr enriched in Cr and Fe2+. Mg and Al diffuse away from primary Chr, leaving a residual Al- and Mg-depleted, Cr-rich Chr and Fe-Chr. Porous Chr is enriched in Zn, Co, Mn, but depleted in Ga, Ni, and Sc, due to the fluid-assisted formation of Chl within the pores of porous chromite. Zn, Co, and Mn from the metamorphic fluids diffuse toward the cores of chromite. The elements Sc, Ti, V, and Ni diffuse from Cr-poor Chr core to Cr-mag rim with increasing Fe3+/(Fe3+ + Fe2+). Late precipitation of Cr-mag occurred under oxidizing and hydrous conditions. The infiltration of fluid produced non-porous Fe3+-rich Chr or Cr-mag along grain boundaries or fractures within chromite. These fluids further promoted the reordering of octahedral sites, enhancing higher mobilization and enrichment of Ga, Ni, Sc, V, and Ti. The infiltration of high oxygen fugacity fluids promoted diffusion of Ga, Ni, Sc, and Ti toward the rims, and diffusion of Zn, Co, and Mn toward the cores. All chromite grains from the Songshugou dunites have low Mg# [molar 100 × Mg/(Mg + Fe2+)] values, and high Cr# [molar 100 × Cr/(Cr + Al)] and Fe3+/(Cr + Al + Fe3+), similar to metamorphic chromite formed under amphibolite-facies conditions. The observed positive Zn–Co–Mn anomalies and negative Ga–Ni–Sc anomalies in chromite, and the crystallization of Chl, are fingerprints of amphibolite-facies metamorphism. The compositions of chromite, the inferred mineral assemblage in which it occurs, and mineral equilibria modeling of the assemblage all suggest amphibolite-facies metamorphic temperatures ranging from 600 °C to 700 °C.

Timing of Orogenic Exhumation Processes of the Qinling Orogen: Evidence From40Ar/39Ar Dating

AbstractThe Qinling Orogenic Belt was formed by long‐lived subduction and collision between the North and South China blocks. Our40Ar/39Ar geochronological data of amphibole, white mica, and biotite reveal different exhumation and cooling histories of the major units in the North Qinling Belt and South Qinling Belt (SQB), constraining the multiple orogenic processes. The exhumation of the Qinling Group in the North Qinling Belt started before 432 Ma, through about 500 °C during 432.3–405 Ma, followed by rapid cooling below 425 °C at 388.9 ± 1.1 Ma and slow cooling through about 300 °C during 333.3–329.5 Ma. A similar exhumation history of the Kuanping Group started from about 383 Ma, cooled below 425 °C at 364.9 ± 1.2 Ma and 300 °C at 341.8 ± 1.3 Ma. The metamorphic fore‐arc sedimentary wedge cooled below 500 °C during 322–311.2 Ma, below 425 °C at 306.3 ± 2.8 Ma, and 300 °C at 249 ± 2.8 Ma. However, the white mica40Ar/39Ar ages of 349.4 ± 5.1 and 363.2 ± 2.5 Ma constrain the syntectonic metamorphic crystallization age of the Devonian Liuling Group in the northern SQB, whereas 228.2 ± 0.7 Ma indicates either a delayed cooling of the Wudang Group in the southern SQB or a second stage of low‐temperature thermal overprint. Together with previous cooling ages and regional geology, our new data support a tectonic model of Early Paleozoic Andean‐type orogeny with subsequent Late Paleozoic continental subduction in the Qinling Orogenic Belt.

Field geology, geochronology, and isotope geochemistry of the Luyuangou gold deposit, China: Implications for the gold mineralization in the eastern Qinling Orogen

Gold deposits are closely related to accretionary processes. Gold mineralization in eastern China, however, is usually > 100 Ma younger than associated collisional orogeny and partly controlled by extensional tectonics. Being controlled by interbedded detachment faults, the Luyuangou hydrothermal deposit is a typical mineralization under extensional tectonic. We present mineral deposit geology, tectonic geology, LA‐ICP MS zircon U–Pb age data, and Sr‐Nd‐Hf isotopic data for the Luyuangou ore vein and porphyritic dyke, with the aim of elucidating the metallogenesis. The morphological features and ages of zircons from the Luyuangou auriferous lode are similar to the anatectic zircons from migmatite within the Qinling Group (a metamorphic complex unit) of the North Qinling Terrane, indicating that the North Qinling Terrane may be one of the provenances. Although the major age peak of 453 ± 13 Ma (MSWD = 3.3, N = 12) cannot constrain the exact metallogenic timing, the data imply a possible link between the Luyuangou metallization and the North Qinling Terrane. Meanwhile, the weighted‐mean age of post‐ore porphyritic dyke postdates the Luyuangou gold mineralization at 121.1 ± 1.5 Ma (MSWD = 0.21, N = 14). The Sr‐Nd‐Hf isotopes of the Luyuangou auriferous lodes and post‐ore porphyritic dyke exhibit prominent differences from those of the Taihua Group and Xiong'er Group and are similar to those of the Qinling Group, implying that the Qinling Group is likely the main ore‐forming provenance of the Luyuangou gold deposit. The S‐Pb isotopes of the main mineralization stage of the Luyuangou gold deposit indicate a distinct deep‐seated source. According to comprehensive analysis, it is proposed that, during the early Mesozoic, the large‐scale continental collision between the Yangtze and the North China Craton involved the underthrusting of the North Qinling Terrane beneath the North China Craton that led to intracontinental folding, thrusting, and thickening without inducing extensive mineralization or magmatic activity. During the late Mesozoic, the subduction of the Pacific Plate induced the instability of the thickened continental crust and the development of the regional extensional structures, large‐scale crust–mantle magmatism as well as deep‐seated or mantle‐derived fluids dissolving materials from the Qinling Group that precipitated at the interbedded detachment faults.

Mylonitized peridotites of Songshugou in the Qinling orogen, central China: A fragment of fossil oceanic lithosphere mantle

The Songshugou mylonitized peridotites within the Qinling Group metamorphic rocks in Central China are distributed in the northern part of the Shang-Dan Suture Zone (SDSZ) and contain abundant dunites and harzburgites. The dunites were intensely deformed and mylonitized converting the coarse-grained type to medium- and fine-grained types which contain prominent lenticular structure and relict olivine (Ol) porphyroclasts. Mineralogical and geochemical compositions suggest that the protoliths of the mylonitized peridotites were coarse-grained peridotites of lithospheric mantle origin. The harzburgites occur as enclaves within mylonitic peridotites in the form of lenses or veins. The orthopyroxenes in harzburgites were formed at the expense of Ol and have similar compositions to those of metasomatized harzburgites, characterized by low Al2O3, CaO and Cr2O3 contents. The harzburgites exhibit the gently U-type REE patterns with enriched incompatible elements (Rb, Ba, Sr, Zr and Hf), suggesting the metasomatic origin. The obvious ductile deformation of the large porphyroclastic orthopyroxene (Opx) suggests that the metasomatism occurred before the deformation. Ductile shearing deformation is indicated by the small fold structures and net-style ductile shearing zones within the Songshugou peridotite massif. The process is also result in the alignment of elongated Ol grains from initially coarse-granular via porphyroclastic to fine-granular texture. The relatively low Fo olivine, together with high Al2O3, and CaO contents and the abnormally low total PGE abundance in the fine-grained dunites suggest the ingress of melt/fluid during the mylonitization. The presences of significant amount of amphibole in the peridotites indicate the ingress of hydrous fluids. In general, the Songshugou peridotites have similar compositional characteristics with peridotites of Oman and Troodos ophiolites which are fragments of oceanic lithosphere mantle. One coarse-grained dunite has a TRD age of 875Ma. Additionally two stages Paleozoic TRD ages are obtained from medium-grained and fine-grained dunites (491Ma and 550Ma; 446Ma and 476Ma). The broadly coeval nature of mylonitization with progressive metamorphism of surrounding amphibolites suggested that the Songshugou peridotites were generated before the early Paleozoic deformation. Our data, combined with the previous work on the surrounding HP/UHP metamorphic rocks, demonstrate that the Songshugou mylonitized peridotites represent fragments of the Neoproterozoic fossil oceanic lithospheric mantle that experienced extensive deformation during the Early Paleozoic subduction processes.

Precambrian tectonic affinity of theNorthQinlingMicrocontinent: Constraints from the discovery ofMesoproterozoic magmatic zircons in theQinlingGroup

This paper presents new results on zircon U–Pb dating of a two‐mica schist from the Qinling Group in the core of the North Qinling Orogenic Belt (NQB), Central China, with the aim of constraining the formation time of the Qinling Group, and the tectonic affinity of the North Qinling Microcontinent. Zircon U–Pb dating indicates that the protolith of the metasedimentary rock in the Qinling Group was deposited later than ~1502 Ma. The Qinling Group in the NQB represents a long‐lived unit, and its formation age can be constrained to the period between the Paleoproterozoic and early Neoproterozoic. The zircon age populations of the studied sample can be divided into two major populations of 1559–1642 and 1470–1548 Ma, and one subordinate population of 1717–1794 Ma, indicating their provenances were mainly attributed to the NQB and subordinately from the North China Craton (NCC) and the Yangtze Craton (YZC). And the distribution of the early Neoproterozoic magmatic events and the Nd–Pb whole‐rock isotopes, zircon Hf isotopes, and trace element compositions of the Precambrian basement rocks in the NQB is significantly different from those in the NCC and YZC, indicating that the NQB cannot be a southern sector of the NCC, or a northern sector of the YZC, and was an independent microcontinent from the Paleoproterozoic to the early Neoproterozoic.

Structural analysis of ductile shear zones in the North Qinling Orogen and its implications for the evolution of the Proto‐Tethys Ocean

The ductile shear zones in the North Qinling Orogen, such as the Luonan‐Luanchuan, Guanpo‐Qiaoduan, Zhuyangguan‐Xiaguan, and Shangdan shear zones, play an important role in the preclosure and postclosure evolution of the Proto‐Tethys Ocean. On the basis of detailed macrostructural/microstructural analysis and quartz C‐axis electron backscattered diffraction fabric analysis, the sense and formation environment of the 4 shear zones have been obtained. The Luonan‐Luanchuan and Guanpo‐Qiaoduan shear zones mainly present as dextral and sinistral shearing under low temperature, respectively. The Zhuyangguan‐Xiaguan shear zone mainly presents as dextral shearing under medium temperature. The Shangdan shear zone underwent an early dextral shearing under medium temperature and a late sinistral shearing under low temperature. The Qinling Group and the Taibai region were intensively affected by the late sinistral shearing of the Shangdan shear zone. According to published metamorphic ages, the 4 shear zones should have been formed during the collision between the North China Block and the South China Block. Due to the opening of the Mianlue and the Paleo‐Tethys oceans since ~380 Ma, the North China Block and the South China Block departed from northern Gondwanaland and drifted northward along transform faults, which led to the arrangement between the North China Block and the South China Block that transited from an east–west direction to a north–south direction. Since ~320 Ma, the scissors‐type closure between the North China Block and the South Qinling Terrane led to the westward extrusion of the North Qinling Orogen, and the dextral shearing of the Luonan‐Luanchuan and the Zhuyangguan‐Xiaguan shear zones and the sinistral shearing of the Guanpo‐Qiaoduan and the Shangdan shear zones.

Stratigraphy and tectonic setting of Laochang massive sulfide deposit in the North Qinling belt, central China

The Qinling orogenic belt in central China is the northernmost orogenic collage within the Tethyan domain, which records the evolution of the Paleo-Tethys Ocean. A suite of volcano-sedimentary rocks containing rare fossils and several VHMS deposits is exposed along the North Qinling belt. These units are separated into the Caotangou, Xieyuguan and Erlangping groups from west to east. Systematic studies on the facies and geochemistry demonstrate that the Caotangou Group represents a bimodal volcanic sequence formed in a backarc setting with massive sulfide horizons closely associated with a siliciclastic-felsic volcanic sequence. SHRIMP and LA-ICP-MS zircon U-Pb data from rhyolite and tuff of the Caotangou Group indicate that the volcanism took place between ca. 440–406Ma, which is similar to the dacite and andesitic basalt of the Xieyuguan Group. The Precambrian xenocrystic zircon grains suggest that the subduction-related crustal source of those volcano-sedimentary rocks in the North Qinling belt is closely related to the Qinling Group. Based on the combination of previous fossil ages, U-Pb dating of volcanic rocks and geochemistry of lavas and sulfides, we suggest that a southward-facing subduction-accretionary system developed along the southern margin of the North China plate during 490–410Ma. Subduction-related calc-alkaline magmatism continued until at least ca. 410Ma before collision of the arc with the Qinling terrane to the south during the Early Devonian led to the continent-continent collision between the Qinling terrane and North China craton in the Late Carboniferous.

Geochronology and geochemistry of the Tianmugou Mo deposit, Dabie Shan, eastern China: Implications for ore genesis and tectonic setting

The Tianmugou molybdenum (Mo) deposit is a newly discovered porphyry deposit in the Dabie Shan of eastern China. The Mo-mineralization is spatially and genetically related to the Tianmushan granite batholith, and mainly occurs as thin molybdenite-bearing pegmatite dykes and numerous hydrothermal veins, with the development of potassic alteration, silicification, sericitization, propylitization, and fluoritization. The Tianmushan batholith is composed of outermost fine-grained K-feldspar granite (Phase 1), medium-grained K-feldspar granite (Phase 2), coarse-grained biotite monzogranite (Phase 3), porphyritic granite (Phase 4), and innermost fine-grained K-feldspar granite (Phase 5). Zircon crystals from the biotite monzogranite yield a weighted average 206Pb/238U age of 123.0±0.8Ma, but those from the porphyritic granite do not yield a good age because of their high contents of Th, U and Pb. Seven molybdenite samples from the ores yield individual Re–Os isotopic ages of 120.5±1.7Ma to 122.5±1.9Ma, which are slightly younger than the U–Pb age for the biotite monzogranite. These ages constrain the ore-forming magmatic-hydrothermal activity at the Tianmugou Mo deposit that took place during the post-collisional tectonism subsequent to the collision between the Yangtze and North China continents, consistent with the 142–111-Ma regional Mo mineralization event in Dabie Shan. The biotite monzogranite and porphyritic granite have high contents of SiO2, K2O and Al2O3, and low contents of TiO2, MgO and CaO, showing a metaluminous to peraluminous high-K calc-alkaline affinity, with significant depletion in Eu, Ba, Sr, P and Ti, and enrichment in Rb, Th, U, Nb, Ta, Hf and Y. The rocks have high initial 87Sr/86Sr ratios of 0.73546–0.89204, and negative εNd(t) values of −11.6 to −11.0, with TDM2(Nd) of 1.81–1.86Ga. Their (206Pb/204Pb)t, (207Pb/204Pb)t and (208Pb/204Pb)t are 17.337–17.554, 15.450–15.465 and 37.992–38.147, respectively. The Sr-Nd-Pb isotopic signatures indicate that the causative granitic intrusions for the Tianmugou deposit mainly originated from partial melting of the Qinling Group during Early Cretaceous post-collisional tectonic extension. The Tianmugou Mo deposit belongs to a collisional- or Dabie-type porphyry Mo system.

Identification of coesite-bearing amphibolite in the North Qinling and its geological significance

The Qinling orogen lying in between the North and South China blocks is a key for understanding the tectonic framework of China continent. Tectonically, the Qinling orogen is divided into the North Qinling Tectonic Belt (NQTB) and the South Qinling Tectonic Belt (SQTB) by the Shangdan Fault Zone. With discoveries and study of the continental subduction ultrahigh pressure (UHP) metamorphic belt in the Dabie-Sulu and the North Qaidam-Altyn orogens in east and west China, respectively, a frequently-asked question is whether or not the Qinling orogen which belongs to the same orogenic system with the Dabie-Sulu orogen in central China has experienced the similar continental subduction and what timing of the subduction is. In fact, several UHP metamorphic outcrops such as the diamond-bearing eclogite and muscovite quartz schist in the Guanpo area, quartz-feldspathic UHP rocks in the Songshugou area and diamond-bering amphibolite in the Qingyouhe area have been reported lately in the Qinling group of the NQTB. However, these UHP findings have not been widely accepted with serious attention. The reason for this situation can be attributed to absence of convincing interpretation for the mineral exsolution structure and the UHP index minerals other than diamond especialy often-seen coesite in the orogens containing the UHP metamorphic bodies in the world.