Early Paleozoic Magmatism In The North Qinling Orogenic Belt And Its Implications For Continental Crust Evolution

Granitoid is the most important part of continental crust, recording the important information of continental crust growth and evolution. Therefore, the origin of granitoid and its geodynamic implications have always been the research focus in the field of solid earth science. The North Qinling orogen is characterized by extensive magmatic activities in Caledonian and has recorded its whole tectonic evolution history in the early Paleozoic, and thus provide an optimal window to unravelling the petrogenesis of granitoids as well as the evolution of continental crust in orogenic belt. In this contribution, we first carried out high-precision zircon U-Pb geochoronology studies on the Caledonian granitoids; And then the corresponding zircon Hf-O isotopes and whole rock Sr-Nd isotopes in combination with whole rock major and trace elements were analyzed to detect the origin and sources of S-and I-type granites; At last, a new model for the early Paleozoic tectonic evolution of the North Qinling orogen was built and the linkage between the continental arc magmatism and continental evolution has been discussed. The main achievements are obtained as follows:(1) SIMS and LA-ICPMS U-Pb zircon data obtained from magmatic zircons indicate that the Piaochi intrusion was emplaced at 473±4Ma, and contained ca.490Ma inherited zircons. The granite samples from the Piaochi intrusion show typical S-type characteristics, such as the relatively moderate aluminum saturation index (1.00-1.08), low zirconium saturation temperatures, occurrences of muscovite, and high zircon δ18O values (8.70 to 9.54%o). The relatively low Sm/Nd (0.13-0.20) and FeOT/MgO ratios (2.21-3.9) suggest that the granites are unfractionated. The geochemical compositions reveal that they were derived from partial melting of a clay-poor, but plagioclase- and biotite-rich psammitic source, similar to metasedimentary rocks outcropping in the North Qinling unit. Moreover, the Piaochi granites are moderately depleted in Eu and heavy rare earth elements, and have relatively high (La/Yb)N values (22.8-56.8), implying a high-pressure, plagioclase-poor and garnet-rich residual assemblage. The Piaochi granites have high and variable initial Sr/Sr ratios (0.72167-0.72511) and low d(t) (-10.23--8.71) values with two-stage Nd model ages (TDm2) of 1.91-2.03 Ga, which are comparable with those of supracrustal materials in the North Qinling unit. In addition, our results show that the ca.470 Ma magmatic zircons display a narrow εHf(t) array within those of the inherited cores. These imply that the Piaochi granites inherited their isotopic characteristics wholly from their sources, and the Hf isotopes might have been differently homogenized during the partial melting. Our new zircon U-Pb ages indicate that the Piaochi granites were emplaced after the climax of the UHP metamorphism, coeval with later retrograde metamorphism during exhumation. Therefore, we suggest that the formation of the early Paleozoic Piaochi intrusion was triggered by the exhumation of the UHP rocks.(2) New SIMS zircon U-Pb dating constrains the emplacement age of the Huichizi pluton at 422±5Ma, coeval with migmatization and high-T granulite-facies metamorphism in the North Qinling unit, registering the northward subduction of the Shangdan oceanic crust. Granites from the Huichizi pluton in the North Qinling (NQ) unit have high Sr/Y and (La/Yb)N ratios similar to adakites. Their relatively high SiO2, K2O and Na2O and very low MgO, Cr and Ni contents are in the range of high-SiO2 adakites and early Archean TTGs and are compositionally similar to experimental melts derived from metabasalt sources. Rock samples from the Huichizi pluton have εNd(t) and zircon εHf(t) values similar to the Neoproterozoic metabasalts in the NQ unit. In combination with their normal mantle-like δ18OZir values these adakites are best explained by partial melting of the Neoproterozoic mafic crustal root due to subduction of the Shangdan ocean. Regional geological data suggests that the crust was probably thickened by an arc-collision process (ca.490 Ma) prior to the emplacement of the Huichizi pluton. Our results confirm that underplating of mafic magma and its subsequent fusion triggered by slab subduction under high pressure conditions could be an important mechanism for the formation of early continental crust.(3) New SIMS and LA-ICPMS zircon U-Pb age dates indicate that the Zaoyuan intrusion was emplaced at 423±5Ma, coeval with the Huichizi pluton. An integrated geochemical studies have shown that the granites from Zaoyuan intrusion belong to I-type granite. They have high SiO2(67.15-73.84 wt.%) contents and high δ18Ozirvalues (6.95-8.25%o) relative to those of the normal mantle, suggesting that they were mainly derived from partial melting of crustal materials. Whole rock Sr-Nd and Hf-O similations have clearly shown that the Zaoyuan intrusion can be explained by main contributions from the juvenile crustal component with minor contributions from the older crustal component. Mixing of the two components must have occurred before the granitic magmas were generated since the zircons (Zone a) crystallized therein are isotopically uniform (δ18O:7.36-7.96‰) and have consistent morphology. The granite sample 10QL15 has relatively high MgO、Cr、Ni contents and depleted Hf compositions compared to those of the granite sample 10QL19, implying that the source region of the former granite might contain more juvenile crustal materials. Therefore, we conclude that the crustal sources of the Zaoyuan intrusion are heterogenous. The CL images of magmatic zircons from the granite sample 10QL15 reveal two growth stages (Zone a and zone b) and contain a grain-scale record of elevated Th、U and δ18O outward. The slightly elevated δ18Ozir within the individual zircons is preferred to be explained by crustal comtamination rather than magma replenishment, which would cause resorption. During the whole evolutionary history of Zaoyuan granitic magmas, there is no evidence showing the addition of mantle materials. It is therefore not necessary to invoke the presence of mixing of the mantle- and sediment-derived melts to produce arc granites possessing isotope mixing characteristics. Instead, our results suggest that the isotopic signatures of I-type granites are likely source-inherited similar to S-type granites and thus imply crustal reworking rather than crustal growth.(4) Combined with the latest research achievements, a revised tectonic model for the origin of Early Paleozoic granitoids from the North Qinling orogen has been proposed: At ～490-486Ma, the eclogite metamorphism of the North Qinling unit occurred due to its northward subduction beneath the Erlangping unit. At the same time or slightly later, the Shangdan ocean began to be subducted beneath the North Qinling unit, as indicated by subduction-related gabbroic intrusions in the Fushui area. At ～480-440Ma, during the exhumation of the UHP metamorphic rocks, the Piaochi intrusion was likely formed by partial melting of the metasediments, which had suffered UHP metamorphism prior to its emplacement. Almost simultaneously, the subducted Shangdan oceanic crust had reached the Erlangping unit at a low angel. Extensive granitic magmas with depleted isotope compositions were then produced through partial melting of the Erlangping arc crust. Thickening of the North Qinling unit crust might have happened during the early Paleozoic via the the arc-continent collision and the subduction of Shangdan ocean beneath its south. At-440-410Ma, slab roll-back results in a southward-younging trend of arc magmas and the transition of the Erlangping arc into a backarc basin. The mafic intrusion in Tongbai area may be generated by incipient rifting of the Erlangping backarc basin. The-430-410Ma magmatism、migmatization and high-T granulite-facies metamorphism appear to record that the Shangdan ocean has retreated to the North Qinling unit by ca.430Ma.(5) Our results have shown that all the early Paleozoic arc granitoids in North Qinling unit have εHf(t)zir values lying between vectors of εHf(t) evolution for juvenile lower crust and older upper crust of the North Qinling unit, suggesting crustal reworking for their formation. This is mainly because the orogenic crust in North Qinling unit has been already thickened prior to the emplacement of the arc magmas. Therefore, we suggest that continental arc granitoids are mainly resulted from crustal reworking in thickened orogenic crust. In addition, the arc granitoids in North Qinling unit have significantly enriched Hf isotope compositions and higher 818Ozir values compared to those exposed in Erangping unit, consistent with the older precambiran basement underlain the North Qinling unit. The systematic differences in isotope compositions between the arc granitoids in North Qinling unit and Erlangping unit imply that the basement affinity has a key influence on the composion of arc magmas intruded therein. We thus propose that the basement affinity should be fully considered while isotopic compostions of arc magmas are used to discuss continental crustal growth.