Late Paleozoic to Mesozoic Intrusions Distribution in the North Sanjiang Orogenic Belt,Southwest China:Evidence from Zircon U-Pb Dating and Geochemistry

A mosaic of terranes or blocks and associated Late Paleozoic to Mesozoic sutures are characteristics of the north Sanjiang orogenic belt （NSOB）. A detailed field study and sampling across the three magmatic belts in north Sanjiang orogenic belt, which are the Jomda-Weixi magmatic belt, the Yidun magmatic belt and the Northeast Lhasa magmatic belt, yield abundant data that demonstrate multiphase magmatism took place during the late Paleozoic to early Mesozoic. 9 new zircon LA-ICP-MS U-Pb ages and 160 published geochronological data have identified five continuous episodes of magma activities in the NSOB from the Late Paleozoic to Mesozoic： the Late Permian to Early Triassic （c. 261-230 Ma）; the Middle to Late Triassic （c. 229-210 Ma）; the Early to Middle Jurassic （c. 206-165 Ma）; the Early Cretaceous （c. 138-110 Ma） and the Late Cretaceous （c. 103-75 Ma）. 105 new and 830 published geochemical data reveal that the intrusive rocks in different episodes have distinct geochemical compositions. The Late Permian to Early Triassic intrusive rocks are all distributed in the Jomda-Weixi magmatic belt, showing arc-like characteristics; the Middle to Late Triassic intrusive rocks widely distributed in both Jomda-Weixi and Yidun magmatic belts, also demonstrating volcanic-arc granite features; the Early to Middle Jurassic intrusive rocks are mostly exposed in the easternmost Yidun magmatic belt and scattered in the westernmost Yangtza Block along the Garze-Litang suture, showing the properties of syn-collisional granite; nearly all the Early Cretaceous intrusive rocks distributed in the NE Lhasa magmatic belt along Bangong suture, exhibiting both arc-like and syn-collision-like characteristics; and the Late Cretaceous intrusive rocks mainly exposed in the westernmost Yidun magmatic belt, with A-type granite features. These suggest that the co-collision related magmatism in Indosinian period developed in the central and eastern parts of NSOB while the Yanshan period co-collision related magmatism mainly occurred in the west area. In detail, the earliest magmatism developed in late Permian to Triassic and formed the Jomda-Wei magmatic belt, then magmatic activity migrated eastwards and westwards, forming the Yidun magmatic bellt, the magmatism weakend at the end of late Triassic, until the explosure of the magmatic activity occurred in early Cretaceous in the west NSOB, forming the NE Lhasa magmatic belt. Then the magmatism migrated eastwards and made an impact on the within-plate magmatism in Yidun magmatic belt in late Cretaceous.

Discovery of the Early Paleozoic Boin Sum-Ordor Sum Island Arc in the Hadamiao Gold Ore District, Inner Mongolia and its Significance to the Evolution of the Paleo-Asian Ocean

Early Paleozoic granodiorite has been identified on the northern margin of the North China craton in the east section of the central-Asian orogenic belt, which was previously known as early Indosinian in age. By using the LA-ICP-MS method, the obtained zircon U-Pb age is 445.6 2.7 Ma, which represents the crystallization age of the granodiorite. The granodiorite near the east of the large-sized Bilihe gold deposit is of the tholeiite series with low potassium. It is quasi-aluminous I-type granite, enriched in sodium （Na2 O/K2O=7.29 9.77） and magnesium （Mg # =0.51 0.67）. The ΣREE value is relatively low, obvious differentiation is shown between LREE and HREE and within LREE, and the Eu anomaly is low and negative （δEu=0.74 0.91）. In the primitive-mantle normalized spider diagrams of trace elements, the granodiorite is relatively rich in LREE and LILE （Ba, Sr, Th）, and strongly depleted in HFSE （Nb, Ta, Ti and P）, which shows features of subduction zone components （SZC）. In the discrimination diagrams of tectonic settings of granite for Rb vs. （Nb＋Y）, Rb vs. （Ya＋Ta）, La/Nb vs. Ba/Nb and Th/Nb vs. Ba/Nb, the granodiorite exhibits typical features of island arc granite. The normalized values of K and Rb are extremely low, while the values of Sr and Eu are very high, which are similar to those of island arc magma that has undergone metasomatism of fluid from the oceanic crust. The granodiorite is relatively depleted in ε Hf （t） （5.1 7.1） and low in ε Hf （t） model ages （1089 921 Ma）. In the ε Hf （t） vs. age （T） diagram, the distribution area of the granodiorite is accordant with the field of the Xing’anling-Mongolia orogenic belt, which indicates that the magmatic sources are mainly the mixture of partial melting of wedged mantle subjected to metasomatism of fluid from the oceanic crust and young substance from the crust. The granodiorite is similar to the felsic arc magma in the Damao Banner, Bate Obon, Boin Sum and Ordor Sum regions, and they altogether constitute an early Paleozoic accretionary island arc magmatic belt on the northern margin of the North China craton. A number of early Paleozoic zircons trapped in late Paleozoic intrusions in the Hadamiao and Bilihe regions and the discovery of the early Paleozoic island arc magmatic belt near the east of the Bilihe gold deposit suggest that the late Paleozoic volcanic-intrusive rocks have a basement of early Paleozoic arc accretionary complexes. This is just the evident of the multiphase subduction and accretion model of the Paleo-Asian Ocean （PAO）. Paleozoic structures and magmas on the northern margin of the North China craton are shown from south to north as the late Paleozoic Andes-type arc magmatic belt in the Inner Mongolia plateau, the Chifeng-Bayan Obo fault and the late and early Paleozoic arc magmatic belt, which shows that after the early Paleozoic arc-continent collisional orogeny and at the stage of the late Paleozoic accretionary orogeny, the PAO plate was likely to continuously pulsate and underthrust beneath the early Paleozoic island arc accretionary complex belt and its front, i.e. the North China craton. During the early Paleozoic collisional orogeny, the PAO plate might not experience large-scale breakup or delamination. The characteristics of the early Paleozoic island arc accretionary complex basement have a significant control on late Paleozoic diagenesis and metallization in the Hadamiao and Bilihe gold concentrated areas.

Devonian alkaline magmatism in the northern North China Craton:Geochemistry,SHRIMP zircon U-Pb geochronology and Sr-Nd-Hf isotopes

The Wulanhada pluton is among the rare suite of Devonian alkaline plutons occurring along the northern margin of the North China Craton（NCC）.The intrusion is mainly composed of quartz-monzonite.Here we report zircon SHRIMP U-Pb data from this intrusion which shows emplacement age of ca.381.5 Ma.The rock is metaluminous with high（Na2O ＋ K2O） values ranging from 8.46 to 9.66 wt.%.The REE patterns of the rocks do not show any Eu anomaly whereas the primitive-mantle-normalized spider diagram shows strong positive Sr and Ba anomalies.The Wulanhada rocks exhibit high initial values of（87Sr/86Sr）t = 0.70762-0.70809,low εNd（t） =-12.76 to-12.15 values and negative values of εHf（t） =-23.49 to-17.02 with small variations in（176Hf/177Hf）,（0.281873-0.282049）.These geochemical features and quantitative isotopic modeling results suggest that the rocks might have been formed through the partial melting of Neoarchean basic rocks in the lower crust of the NCC.The Wulanhada rocks,together with the Devonian alkaline rocks and mafic-ultramafic complex from neighboring regions,constitute a post-collisional magmatic belt along the northern NCC.

The Late Cretaceous Crustal Magmatism of the Geza Arc Metallogenic Belt in Yunnan Province,and Zircon Ages and Hf Isotopic Evidence of the Porphyry Cu-Mo Mineralization

Objective The Geza arc in Yunnan Province,located in the southern Yidun arc,is an important part of the Sanjiang tectonic-magmatic belts in southwestern China and is a newly discovered copper polymetallic ore-concentrating district.Recent studies show that the newly discovered Yanshanian porphyry Cu-Mo polymetallic mineralization superimposed in the Indosinian porphyry copper belt in this area.

Geochemistry of Enclaves and Host Granitoids from the Kashan Granitoid Complex, Central Iran: Implications for Enclave Generation by Interaction of Cogenetic Magmas

The major and trace elements and Sr-Nd-Pb isotopes of Miocene host granitoid rocks and their mafic microgranular enclaves（MMEs） were studied to understand the petrogenesis of MMEs in the Kashan complex, which is part of the Urumieh-Dokhtar magmatic belt（Iran）. The host rocks consist of quartz-diorite and tonalite associated with a dioritic intrusion. The enclaves show microgranular texture and the same mineralogy as their respective host with plagioclase, quartz and biotite. MMEs have a diorite to quartz-diorite composition and show geochemical characteristics mostly between their granitoid host and the diorite intrusion. Chondrite-normalized REE patterns of all samples are moderately fractionated [（La/Yb）N=2.1 to 12.9]. The MMEs display in part small negative Eu anomalies（Eu/Eu＊=0.54 to 0.99）, with enrichment of LILE and depletion of HFSE. The enclaves show emplacement depth of -4 to 6 km which is comparable with the host rocks. Moreover, the Hornblende-plagioclase equilibrium temprature of MMEs yields average temperatures of 795℃ which is slightly higher than the host ones. Identical mineral compositions and Nd-Sr-Pb isotopic features of MME-host granitoid pairs indicate interactions and parallel evolution of MME and enclosing granitoid in the Kashan plutons. Additionally, the geochemical and isotopic investigations of host and dioritic intrusions suggest a common source for their genesis. A thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower crust to generate Kashan granitoid rocks.