Mg/Ca,Ba/Ca,and S/Ca ratios as environmental and growth proxies for bivalve shells from the Haima cold seep,South China Sea

Bivalve shell fossils,cemented by authigenic carbonates,are widely spread in the Haima cold seep,Qiongdongnan Basin of the South China Sea(SCS).In this study,we examined an element profile of Gigantidas platifrons in the Haima cold seeps at a water depth of 1381 m.Based on the scanning electron microscope(SEM)analyses,the prismatic layer and nacreous layer were identified,which are characterized by prismatic structure and stratified structure,respectively.In addition,the profile can be subdivided into two parts:altered and unaltered zones.Laser inductively coupled plasma mass spectrometry(LA-ICP-MS)mapping shows that the element concentrations of the altered zones were influenced by the authigenic carbonate rocks,whereas the element concentrations of unaltered zones remain stable.In-situ X-ray diffraction(XRD)analyses show that the mineral constituent of the prismatic layer is mainly composed of aragonite.Along with the growth profile,Mg/Ca ratios of unaltered zones have minor variations,ranging 0.72-0.97 mmol/mol(mean=0.87 mmol/mol),with estimated temperatures of 3.8-4.1℃,indicating that the temperature of the surrounding seawater remains constant and agree with the measured data of 3.9℃which was conducted by a conductivity-temperature-depth system(CTD).The minor variations of Ba/Ca ratios(0.01-0.06 mmol/mol;mean=0.04 mmol/mol)indicate a relatively stabilized salinity of the surrounding seawater.S/Ca ratios show large variations of 0.04-4.15 mmol/mol(mean=1.37 mmol/mol).S/Ca ratios have regular variations which generally correspond to the variations of the Mg/Ca ratios,highlighting that the S/Ca ratios of bivalve shells show the potential to reflect the growth rate of the Gigantides.However,further studies should be carried out on the understanding of the links between the S/Ca ratios and seepage intensity of cold-seep fluids.

A newly Found Biotite Syenogranite in the Huangshaping Polymetallic Deposit, South China: Insights into Cu Mineralization

The W-Sn-Mo-Cu-Pb-Zn polymetallic mineralization in the Huangshaping Deposit is associated with different types of Jurassic granitic intrusions such as quartz porphyry, granophyre and granite porphyry. In this study, we carried out detailed mineralogical, petrological, geochemical, zircon U-Pb geochronological and Hf isotopic studies on a newly found, Cu-mineralization-related finegrained biotite syenogranite at Huangshaping. This syenogranite is mainly composed of quartz （40%）, K-feldspar （30%）, plagioclase （15%） and biotite （5%）, enriching in SiO2 （75.58 wt.%-76.02 wt.%）, K20 （5.24 wt.%-5.78 wt.%）, AI203 （12.23 wt.%-12.46 wt.%）, Rb （405 ppm-504 ppm）, U （18.5 ppm-40.7 ppm）, Th （47 ppm-52.3 ppm）, Nb （49.6 ppm-57.5 ppm）, and Cu （54 ppm-288 ppm） whereas depleting in P205 （0.01 wt.%）, TiO2 （0.05 wt.%-0.06 wt.%）, Ba （5 ppm-13.1 ppm）, Sr （6.8 ppm-23.9 ppm） and Eu （0.04 ppm-0.05 ppm）, with A1-type alkaline granite characteristics. Magmatic and hydrothermal zircons were separated from a Cu-mineralized syenogranite sample and yielded the ^206Pb/^238U concordant ages of 156.5±1.8 and 154.4±2.9 Ma, representing the intrusive age of the pluton and probable Cu-mineralization time at the Huangshaping Deposit, respectively. Zircon Hf isotopes suggest a crust-derived （partial melting of Paleoproterozoic basement） magma for the syenogranite, with some degree of crust-mantle interaction. We infer that the syenogranite is a transitional phase between the granophyre and the granite porphyry, not only contributed greatly to the Cu mineralization but also provided W-Sn polymetallic ore-forming metals for the Huangshaping Deposit.

华夏板块发现冥古宙捕掳锆石

由于缺少冥古宙岩石的证据,对地球冥古宙地壳演化模式仍缺乏深入认识.锆石作为火成岩中常见的副矿物,由于其独特的物理化学稳定性,在研究岩石成因以及地壳演化等方面发挥着重要作用.因此,本文对华夏板块汤湖岩体中新发现的冥古宙捕掳锆石开展了一系列U-Pb年代学和Lu-Hf同位素地球化学研究.结果表明,该锆石具有明显的核幔结构,核部年龄为4102±12 Ma且具有较高的Th/U比(0.48)和ε_(Hf)(t)值(0.07),证实了该捕掳锆石属岩浆成因且形成于冥古宙.本文提出了两种假说且均可解释该锆石的成因:(1)其由4200Ma的亏损地幔派生出的岩石在4100 Ma经历再造;和(2)其由原始地幔在4100 Ma派生出.但是不管是哪种模式都指示了地球在冥古宙存在新生地壳的生长事件.