Petrogenesis and Tectonic Significance of the Xiuwacu Two-Period Magmatism in Geza Arc of Yunnan Province： Constraints from Lithogeochemistry, Zircon U-Pb Geochronology and Hf isotopic Compositions

Objective The Late Cretaceous Xiuwacu ore-bearing porphyry is located in the Geza area of southern Yidun arc, SW China. In this area, the rock mass is mainly composed of three lithofacies： biotite granite porphyry, monzonitic granite and light alkali feldspar granite. As a part of the Yidun arc, the Geza arc has common structure and temporal- spatial evolution with the ~idun arc, which has experienced three stages of oceanic crust subduction, collision orogeny and intracontinent convergence stages. The molybdenite ores in the area are mainly hosted in monzonitic granite-porphyry and structural fracture zone, and the ore bodies are strictly controlled by faults. In recent years, great geological prospecting results have been achieved in Xiuwacu, and the deposit has reached a medium scale. However, there are few researches on the metallogenic porphyry. Based on the previous research, we determined the rock-forming and ore-forming age of the porphyry, and found that there were two stages of magmatism intrusion in Xiuwacu： Indosinian and Yanshanian. We also discussed the geochemical characteristics and source area of the rocks in the area.

Study on lithogeochemistry of Middle Jurassic basalts from southern China represented by the Fankeng basalts from Yongding of Fujian Province

There exists an E-W trending Middle Jurassic volcanic zone in southern China. The Fankeng basalts in the Yongding basin of Fujian Province are considered to be a typical example. The Fankeng basalts have TiO2 contents in the range of 1.92%-3.21%. They are classified as high-Ti basalts. They also have higher total Fe (averaging FeO* = 11.09%). The Middle Jurassic Fankeng basalts from southwestern Fujian have obvious distinctive lithogeochemical features from early Cre- taceous basalts from southeastern coast of China. They have higher HFSE, such as Th, Nb, Ta, Zr and Ti. Their element ratios related with HFSE, such as Zr/Ba, La/Nb, La/Ta ,Zr/Y, Ti/Y, Ba/Nb, K/Ti and Rb/Zr are similar to those of OIB. The most samples have ε Nd(T) of-0.70-0.24, which are near chondrite. Some samples have higher ε Nd(T) of 1.87-3.55.Therefore, these basaltic magmas might be derived from depleted asthenospheric mantle. The lithogeochemical characteristics of the Fankeng basalts may be caused by interaction between asthenosphere and lithosphere at the time. The (Early-) Middle Jurassic basalts and gabbros from southeastern Hunan, southern Jiangxi and northern Guangdong provinces show similar geochemical features to those of the Fankeng basalts from the Yongding of Fujian. Occurrence of these OIB-type basalts in the area may be regarded as the petrological mark of upwelling of asthenosphere at the time. Upwelling of asthenosphere has led to tectonic extension and the formation of rifted basin in the area.

Isotope Geochronologic and Geochemical Constraints on the Magmatic Associations of the Collisional Orogenic Zone in the West Kunlun Orogen, China

The Jiajiwaxi pluton in the southern portion of the West Kunlun Range can be divided into two collision-related intrusive rock series, i.e., a gabbro-quartz diorite-granodiorite series that formed at 224±2.0 Ma and a monzonitic granite-syenogranite series that formed at 222±2.0 Ma. The systematic analysis of zircon U-Pb geochronology and bulk geochemistry is used to discuss the magmatic origin （material source and thermal source）, tectonic setting, genesis and geotectonic implications of these rocks. The results of this analysis indicate that the parent magma of the first series, representing a transition from I-type to S-type granites, formed from thermally triggered partial melting of deep crustal components in an early island-arc-type igneous complex, similar to an I-type granite, during the continental collision orogenic stage. The parent magma of the second series, corresponding to an S-type granite, formed from the partial melting of forearc accretionary wedge sediments in a subduction zone in the late Palaeozoic-Triassic. During continued collision, the second series magma was emplaced into the first series pluton along a central fault zone in the original island arc region, forming an immiscible puncture-type complex. The deep tectonothermal events associated with the continent-continent collision during the orogenic cycle are constrained by the compositions and origins of the two series. The new information provided by this paper will aid in future research into the dynamic mechanisms affecting magmatic evolution in the West Kunlun orogenic belt.

A geochronological and geochemical study on the granodiorite porphyry and its implication for the mineralization in the Dayaoshan metallogenic belt,Southeastern China

A systematic study combining U-Pb zircon dating,lithogeochemical and Sr-Nd isotopic analyses was carried out upon the Xinping granodiorite porphyry in the Dayaoshan metallogenic belt to understand its petrogenesis and tectonic significance.LA-ICP-MS U-Pb zircon dating yielded a 442.7±5.8 Ma age,indicating that the granodiorite porphyry was emplaced during the Llandovery Silurian of the Early Paleozoic.The granodiorite porphyry shares the same geochemical characteristics such as Eu negative anomaly as other syn-tectonic granite plutons in the region,including the granodiorite porphyry in Dawangding and granite porphyries in the Dali Cu-Mo deposit and Longtoushang old deposit,indicating a similar magma evolution process.The Xinping granodiorite porphyry has high contents of SiO2(67.871.8%)and K2O(1.78-3.42%)and is metaluminous-peraluminous with A/CNK ratios ranging from 0.97 to 1.06,indicative of high-potassium calc-alkaline to calc-alkaline affinity.It is a I-type granite enriched in large ion lithophile elements Rb,Sr,while depleted in Ba and high field-strength element Nb.Tectonically,a collision between the Yunkai Block from the south and the Guangxi Yunnan-North Vietnam Block from the north during the Early Paleozoic was followed by uplifting of the Dayaoshan terrane.The Xinping granodiorite porphyry was likely emplaced during the collision.Sr-Nd isotopic analyses show that the granodiorite porphyry has initial 87Sr/86Sr ratios(Isr)of 0.7080-0.7104,εNd(t)range from-0.08 to-4.09,and t2DM between 1.19 and 1.51 Ga,well within the north-east low-value zone of the Cathaysia block,indicating a Paleoproterozoic Cathaysia basement source and an involvement of under plating mantle magma.Field observations,geochronological data,and 3D spatial distribution all lead to the conclusion that the Early Paleozoic Xinping granodiorite porphyry does not have any metallogenic and temporal relationships with the Xinping gold deposit(which has a Jurassic-Early Cretaceous age based on previous studies)but a close metallogenic relation to W-Mo mineralization.

Origin and Geodynamic Implications of Concealed Granite in Shadong Tungsten Deposit, Xinjiang, China： Zircon U-Pb Chronology, Geochemistry, and Sr-Nd-Hf Isotope Constraint

Shadong deposit is the first large-scale tungsten deposit found in the East Tianshan orogenic belt, and the geologic characteristics of the deposit indicate that the deeply concealed granite body is ge- netically related with the mineralization. The LA-ICPMS U-Pb age of zircons from the Shadong concealed granite obtained in this research is 239±2.0 Ma, belonging to the Middle Triassic. The whole rock samples are metaiuminous to slightly peraluminous （A/CNK=0.95-1.02） with low contents of SiO2 （64.0 wt.%-68.5 wt.%） and low K2O/Na2O ratios （0.73-0.96）. The samples reveal enrichment of K, Rb, Th and depletion of Nb, Ta, P, Ti and have a negative slope from La to Lu （LaN/YbN=16.29-36.8） with weak negative Eu anom- aly （Eu/Eu＊= 0.71-0.82）. Initial ^87Sr/^86Sr ratios of whole rock range of 0.706 59-0.707 75, eNd（t） values range from -1.77 to -2.53 and εnf（t） values of zircon are between 2.54 and 4.90. The lithogeochemistry and Sr-Nd-Hf isotopic characteristics revealed that the concealed granite in Shadong tungsten deposit is I-type granite, and occurs in an intraplate tectonic setting. The magma mixing during intraplating of mantle de- rived magma intruding into the crust in Indosinian Period is the major formation mechanism of the granite. Of which, the proportion of mantle derived magma ranges from 58% to 60%, and the crustal materials are mainly the metamorphic basement of Xingxingxia Group of Mesoproterozoic Changcheng System, which may orovide the main source of ore forming metals of Shadong tungsten deposit.