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The Geochemical Characteristics, SHRIMP Dating and Geological Significance of Leucocratic Rocks in Pillow Lava in Xiaowangjian, North Qinling
The crustal evolution of the Yangtze block and its tectonic affinity to other continents of Rodinia and subsequent Gondwana have not been well constrained. Here, we present new U-Pb ages and Hf isotopes of detrital zircons from the late Neoproterozoic to early Paleozoic sedimentary rocks in the northwestern margin of the Yangtze block to provide critical constraints on their provenance and tectonic settings. The detrital zircons of two late Neoproterozoic samples have a small range of ages (0.87−0.67 Ga) with a dominant age peak at 0.73 Ga, which were likely derived from the Hannan-Micangshan arc in the northwestern margin of the Yangtze block. In addition, the cumulative distribution curves from the difference between the depositional age and the crystalline age (CA−DA) together with the mostly positive εHf(t) values of these zircon crystals (−6.8 to +10.7, ∼90% zircon grains with εHf[t] > 0) suggest these samples were deposited in a convergent setting during the late Neoproterozoic. In contrast, the Cambrian−Silurian sediments share a similar detrital zircon age spectrum that is dominated by Grenvillian ages (1.11−0.72 Ga), with minor late Paleoproterozoic (ca. 2.31−1.71 Ga), Mesoarchean to Neoarchean (3.16−2.69 Ga), and latest Archean to early Paleoproterozoic (2.57−2.38 Ga) populations, suggesting a significant change in the sedimentary provenance and tectonic setting from a convergent setting after the breakup of Rodinia to an extensional setting during the assembly of Gondwana. However, the presence of abundant Grenvillian and Neoarchean ages, along with their moderately to highly rounded shapes, indicates a possible sedimentary provenance from exotic continental terrane(s). Considering the potential source areas around the Yangtze block when it was a part of Rodinia or Gondwana, we suggest that the source of these early Paleozoic sediments had typical Gondwana affinities, such as the Himalaya, north India, and Tarim, which is also supported by their stratigraphic similarity, newly published paleomagnetic data, and tectono-thermal events in the northern fragments of Gondwana. This implies that after prolonged subduction in the Neoproterozoic, the northwestern margin of the Yangtze block began to be incorporated into the assembly of Gondwana and then accept sediments from the northern margin of Gondwanaland in a passive continental margin 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 εNd(t) (−10.4) and εHf(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 °C) 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 postcollision setting may be a potential model for the genesis of granites in collisional orogenic belt.
The relationship between the Shangdan and Erlangping suture zone (ESZ) and the initiation stretch time of the Early Paleozoic back‐arc system in the Qinling Orogen still remains unclear. This paper reports the chronological and geochemical data of the Miaogou mafic intrusion along the southern margin of the Shangdan suture. The pyroxenite in the mafic intrusion has a zircon U–Pb age of 485 ± 5 Ma (Mean Standard Weighted Deviation (MSWD) = 0.08, n = 18). It has low SiO2 (45.41–46.02 wt%), K2O (0.42–0.44 wt%), and TiO2 (0.73–0.83 wt%) contents, high MgO (13.9–15.13 wt%) contents with high Mg# (74–77). In combination with its high clinopyroxene portion, the pyroxenite represented the accumulates from the mafic magma. Some sieve rims of the clinopyroxene indicate that the primitive pyroxenite was metasomatized by subduction‐related fluids. However, their consistent trace element and rare earth element (REE) patterns suggest that the metasomatism process has insignificant effects on their geochemical features. Zircons from the pyroxenite have positive εHf(t) (+7.18 to +11.75) values, reveal a depleted source region. In combination with its high V (322–1,093 ppm) contents, low Ti/V (10–15) ratios and smooth REE patterns, we propose that the pyroxenite was derived from metasomatized depleted mantle wedge in a back‐arc basin. In considering the duration limit (<475 Ma) of the back‐arc basin along the ESZ, we argued that the ~485 Ma Miaogou pyroxenite represented the initiation back‐arc basin in the Shangdan suture, which might represent the initial rifting of the North Qinling microcontinent induced by the northward subduction of the Shangdan Ocean.
The NWW-striking North Qilian Orogenic Belt records the Paleozoic accretion–collision processes in NW China, and hosts Paleozoic Cu–Pb–Zn mineralization that was temporally and spatially related to the closure of the Paleo Qilian-Qinling Ocean. The Wangdian Cu deposit is located in the eastern part of the North Qilian Orogenic Belt, NW China. Copper mineralization is spatially associated with an altered early Paleozoic porphyritic granodiorite, which intruded tonalites and volcaniclastic rocks. Alteration zones surrounding the mineralization progress outward from a potassic to a feldspar-destructive phyllic assemblage. Mineralization consists mainly of quartz-sulfide stockworks and disseminated sulfides, with ore minerals chalcopyrite, pyrite, molybdenite, and minor galena and sphalerite. Gangue minerals include quartz, orthoclase, biotite, sericite, and K-feldspar. Zircon LA–ICPMS U–Pb dating of the ore-bearing porphyritic granodiorite yielded a mean 206Pb/238U age of 444.6±7.8 Ma, with a group of inherited zircons yielding a mean U–Pb age of 485±12 Ma, consistent with the emplacement age (485.3±6.2 Ma) of the barren precursor tonalite. Rhenium and osmium analyses of molybdenite grains returned model ages of 442.9±6.8 Ma and 443.3±6.2 Ma, indicating mineralization was coeval with the emplacement of the host porphyritic granodiorite. Rhenium concentrations in molybdenite (208.9–213.2 ppm) suggest a mantle Re source. The tonalities are medium-K calc-alkaline. They are characterized by enrichment of light rare-earth elements (LREEs) and large-ion lithophile elements (LILEs), depletion of heavy rare-earth elements (HREEs) and high-field-strength elements (HFSEs), and minor negative Eu anomalies. They have εHf(t) values in the range of +3.6 to +11.1, with two–stage Hf model ages of 0.67–1.13 Ga, suggesting that the ca. 485 Ma barren tonalites were products of arc magmatism incorporating melts from the mantle wedge and the lithosphere. In contrast, the 40-m.y.-younger ore-bearing porphyritic granodiorite is sub-alkaline and peraluminous. They are enriched in LREEs and LILEs, depleted in HFSEs, and show weak negative Eu anomalies. They display εHf(t) values of captured or inherited zircons in the range of +8.5 to +10.0, and younger two–stage Hf model ages of 0.78 Ga and 0.86 Ga, similar to those of ca. 485 Ma tonalite. The ca. 445 Ma zircons have εHf(t) values of –2.1 to +9.9, with two–stage Hf model ages of 0.75–1.27 Ga. Moreover, they have relatively high oxygen fugacity than that of the precursor barren tonalite. The ca. 445 Ma magmas at Wangdian thus formed in a subduction setting, and incorporated melts from the subduction-modified lithosphere that had previously been enriched by additions of chalcophile and siderophile element-rich materials by the earlier magmatism and metasomatism during the Paleo Qilian-Qinling Ocean subduction event.
As the eastern part of the Central Orogenic Belt, Qinling-Dabie Orogenic Belt has a complicated tectonic-sedimentary history. On the basis of a systematic research on the lithostratigraphy, fossil assemblages, isotopic geochronology and tectonics of 4 secondary and 13 three-level tectonic units, this paper divides the study area into 18 types of sedimentary basins, and discusses its tectonic-sedimentary evolution from Neoproterozoic to Mesozoic: (1) Neoproterozoic-Early Paleozoic: North Qinling was magmatic arc and forearc basin; South Qinling experienced intracontinental rift to interplatform basin-platform to continental-margin rift; Dabie-Sulu experienced intracontinental rift to interplatform basin-platform. (2) Late Paleozoic: North Qinling was marine-continental alternation epeiric sea; Mianlüe oceanic basin opened in the Devonian; South Qinling was backarc shelf and interplatform basin-platform at the same time. (3)Triassic: continental collision orogeny, stage of foreland basins. (4) Jurassic-Cretaceous: stage of fault basins and depressional basins.
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