Permian volcanisms in eastern and southeastern margins of the Jiamusi Massif, northeastern China: zircon U-Pb chronology, geochemistry and its tectonic implications

LA-ICP-MS zircon U-Pb dating and geochemical data for the Late Paleozoic volcanic rocks from eastern and southeastern margins of the Jiamusi Massif are presented to understand the regional tectonic evolution. Zircons from eight representative volcanic rocks are euhedral-subhedral in shape and display striped absorption and fine-scale oscillatory growth zoning as well as high Th/U ratios （0.33-2.37）, implying a magmatic origin. The dating results show that the Late Paleozoic volcanic rocks in the study area can be divided into two stages, i.e., the Early Permian （a weighted mean 205^Pb/238^U age of 288 Ma） and the Middle Permian volcanisms （a weighted mean 205^Pb/238^U age of 268 Ma）. The former is composed mainly of basalt, basaltic-andesite, andesite and minor dacite. They are characterized by low SiO2 contents, high Mg^# （0.40-0.59）, enrichment in Na （Na2O/K2O = 1.26-4.25） and light rare earth elements （LREEs）, relative depletion in heavy rare earth elements （HREEs） and high field strength elements （HFSEs）, indicating that an active continental margin setting could exist in the eastern margin of the Jiamusi Massif in the Early Permian. The latter consists mainly of rhyolite and minor dacite with high SiO2 （77.23%-77.52%）, low MgO （0.11%-0.14%）, enrichment in 1（20 （Na2O/K2O ratios 〈 0.80） and Rb, Th, U and depletion in Eu, Sr, P and Ti, implying a crust-derived origin. Therefore, it is proposed that the Middle Permian volcanic rocks could have formed under the collision of the Jiamusi and the Khanka Massifs.     

In situ U-Pb dating of xenotime by laser ablation （LA）-ICP-MS

Xenotime is an ideal mineral for U-Th-Pb isotopic dating because of its relatively high U and Th contents, but typically low concentration of common Pb. These characteristics, and the fact that it is widespread throughout various types of rocks, suggest that the U-Th-Pb dating of xenotime has broad applications. Studies of U-Pb dating on xenotime by ion microprobe (such as SHRIMP) have increased in recent years, whereas studies by laser ablation (LA)-ICP-MS are still rare. In this study, we developed a technique for U-Pb dating of xenotime using the 193 nm ArF laser-ablation system and Agilent 7500a Q-ICP-MS. To evaluate the reliability of our method, a xenotime standard, BS-1, was analyzed and calibrated against another xenotime standard, MG-1. The weighted mean 206 Pb/ 238 U ages of 510.1 ± 5.2 Ma (2 n = 21), 509.8 ± 4.3 Ma (2 n = 21) and 510.0 ± 4.6 Ma (2 n = 21) were obtained using beam diameters of 16, 24 and 32 m, respectively. These ages are identical to those determined by ID-TIMS method (weighted mean 206 Pb/ 238 U age of 508.8 ± 1.4 Ma), which supports the reliability of our LA-ICP-MS method. We also analyzed xenotimes in leucogranites from South Tibet and granites from Xihuashan in southern China, and obtained accurate and precise ages. Nevertheless, we observed systematic differences in Pb/U fractionation among xenotime, monazite and zircon. The matrix-effect resulted in either under-correction or over-correction of fractionation, and thus led to inaccurate ages. Thus, a matrix-matched material is required for U-Pb dating of xenotime by LA-ICP-MS.     

Repeated modification of lithospheric mantle in the eastern North China Craton: Constraints from SHRIMP zircon U-Pb dating of dunite xenoliths in western Shandong

Four dunite xenoliths from the Tietonggou intrusion of western Shandong,China,were subjected to SHRIMP zircon U-Pb dating to constrain timing of the North China Craton(NCC) destruction,a topic of much controversy.Cathodoluminescence images revealed that 15 of the 18 zircon grains from the xenoliths display striped absorption.The rest showed oscillatory growth zoniation.All the zircons had variable contents of Th(49-3569 ppm;average,885 ppm) and U(184-5398 ppm;average,1277 ppm),and variable Th/U ratios(0.15-2.04).These zircon characteristics indicate a magmatic origin.The zircon age data can be divided into five groups:131-145,151-164,261-280,434-452,and 500-516 Ma.Group I(131-145 Ma) is consistent with timing of formation of the Tietonggou high-Mg diorites.Group II(151-164 Ma) is similar in age to Middle-Late Jurassic magmatism in the eastern NCC,which included both mantle-derived and intensive crust-derived magmatism.Group III(261-280 Ma) is similar in age to the Emeishan large igneous province,and Group IV(434-452 Ma) is similar in age to Paleozoic high-silica magmatism in the eastern NCC.Group V(500-516 Ma) may correspond to the global Pan-African event.Results indicate repeated modification of lithospheric mantle in the eastern NCC,and suggest that the most intensive modification occurred in the late Mesozoic(131-164 Ma).     

Comments on the planation surface once more

The authors reviewed the research of planation surface in recent years and commented on the development of method and theory about the planation surface. Some aspects of research, such as the restrictive planation surface, the karst planation surface, the fossil planation surface, the dating of weathering crust, the research of weathering crust and environmental evolution, extremely enriched the method and theory of the planation surface. Besides, the authors pointed out that we must take further steps to study the following issues: the deformation of planation surface, the original height of planation surface, the recognition and contrast of the planation surface. The weathering crust and planation surface should be regarded as a whole, and it will still be a principal aspect in the future.     

Oxygen isotopic compositions of zircons from pyroxenite of Daoshichong,Dabieshan: Implications for crust-mantle interaction

Oxygen isotopic compositions of zircons from pyroxenite (～145 Ma) of Daoshichong, Dabieshan have been measured by an ion microprobe. Both within the single grain and among different grains, oxygen isotopic ratios are homogeneous, δ 18O = (7.66‰±0.46)‰ (1 SD, 1 σ = 0.10, n = 22). High δ 18O values indicate that the mantle-derived parent magma of Daoshichong pyroxenite have undergone interaction with crustal materials. Combing with other geochemical constraints, the way of crust-mantle interaction is suggested to be source mixing other than crustal contamination. The time interval between crust-mantle interaction and formation of the parent magma of Daoshichong pyroxenite is less than several million years. The crustal component involving in crust-mantle interaction is mafic lower crust, and the parent magma of pyroxenite possibly contain large proportion (>37%) of such lower crust.     

<Emphasis Type="Italic">In-situ</Emphasis> trace element analyses of zircons from Dabieshan Huangzhen eclogite: Trace element characteristics of eclogite-facies metamorphic zircon

The internal structures of zircons in eclogite from Huangzhen have been studied by cathodoluminescence (CL) microscopy. Two growth stages were distinguished: protolith magmatic cores and metamorphic overgrowth rims. These different domains were analyzed for trace elements using LAM-ICP-MS. The protolith and the overgrowth zircons have different trace elements characteristics. The trace element contents of protolith zircons are high and very variable. The overgrowth zircons show a typical trace element feature of equilibrium with garnet, e.g. low contents of HREE (132.2–197.6 μg/g) and small differential degree of HREE ((Yb/Gd)_CN=8.6–11.9). The contents of Nb, Ta and the ratio of Nb/Ta are lower in the metamorphic domains (0.5–1.4 μg/g, 0.7–1.5 μg/g, 0.3–1.3, respectively) than in the protolith domains (3.8–19.7 μg/g, 2.7–12.1 μg/g, 1.0–4.6, respectively). This is the first time to give the evidence that the metamorphic zircon equilibrates with the rutile, which formed during the peak metamorphic stage. The REEs and other trace elements data demonstrate that the metamorphic zircons overgrow in the eclogite-facies conditions. The trace element composition of zircon can therefore give new ways to constrain their formation conditions.     

Timing of Holocene sea-level highstands by mass spectrometric U-series ages of a coral reef from Leizhou Peninsula, South China Sea

Systematic mass spectrometric 230Th ages are reported for a Porites coral reef terrace from Dengloujiao,Leizhou Peninsula, South China Sea. Seven episodes of coral growth were recognized in this terrace: 7125±96, 6764±29,5826±37, 5006±54, 2543±24, 1915±15, and 1513±22 calendar years before present (cal. aBP). 50% of the coral age population fall between 7200 and 6600 cal. aBP, marking post-glacial stabdization of global sea level. Considering the facts that ( i ) Dengloujiao reef fiat was measured at 1.6-2.5m above modern tidal datum plane; (ii) modern Porites corals in the South China Sea are living at least ～1 m below the modern tidal datum plane; (iii) the top 20-30 cm of the reef was eroded; and (iv) crustal subsidence in the region since mid-Holocene was negligible, we conclude that the above age groups record at least two major periods (7200-5000 and 2500-1500 cal. aBP) of high sea-level at least 2.9-3.8 m above the present-day level.     

Geological and chronological evidence of Indo-Chinese strike-slip movement in the Altyn Tagh fault zone

A set of granitic and amphibole mylonite are exposed in the Altyn Tagh fault zone. The preliminary study shows that these rocks are the product of the syntectonic anatexis in the process of the left-lat- eral strike-slip shear, and are the result of the ductile transpression. There are two types of zircon sorted from the mylonite formed with synshear anatexis. Among them, one is the anatectic long columnar zircon and another is the residual metamorphic sub-rounded columnar zircon. Two groups of age for single zircon measured by ion microprobe (SHRIMP) are obtained: one is 461-547 Ma for the sub-rounded columnar residual metamorphic zircon, and the other is 239-244 Ma for the long-columnar anatectic zircon. This type of zircon is directionally spread in rock, and the long axis direction of its crystal is identical to that of stretching lineation, representing the direction of tectonic stress in the process of the strike-slip. 40Ar-39Ar age of the directional growth hornblende in the same mylonite sample measured is 223-226 Ma. The above geological and chronological data prove that the syntectonic anatexis occurred during Indo-Chinese Epoch, showing that the strong strike-slip movement occurred in the Altyn Tagh fault zone at least as early as Indo-Chinese Epoch.     

40Ar-39Ar dating and geological implication of auriferous altered rocks from the middle-deep section of Q875 gold-quartz vein in Xiaoqinling area, Henan, China

The 40Ar-39Ar age method is employed in this work to analyze the aurif erous altered rocks from the middle-deep section of the Q875 gold-quartz vein in the Xiaoqinling area, and the results show that the main gold deposition of the Q875 occurred in ca. 128-126 Ma. As a typical gold-bearing quartz vein in this gold-rich area, the age data obtained from the Q875 also constrain on the metal logenic time of the lode gold deposits developed in the same geological settings . This geochronological study supplies new evidence for further understanding the timing of gold mineralization, the genesis of gold deposits and the geodynamic settings in Xiaoqinling area.     

冲绳海槽火山岩同位素特征及铀系组分法测年

借助多种分析方法,对采自冲绳海槽北段和中段的火山岩进行了常量元素、稀土元素、Sr-Nd同位素组成分析及铀系组分法年龄测定,在此基础上对此地区的海槽岩浆的起源、演化及岩石类型进行了研究。结果表明:(1)海槽晚更新世(大约10万年)以来有一期广泛的酸性火山喷发活动;(2)受板块俯冲的控制,此期火山活动年龄按照与琉球岛弧距离的由近到远而由新到老;(3)海槽北、中段岩浆演化过程相似,其下很可能存在有巨大的、统一的岩浆房,但与海槽南段岩浆作用差异明显,岩浆物质很可能来源于PREMA地幔源区;(4)此地区的黑色浮岩为钙碱性系列流纹岩,岩石化学组成与海槽产出的白色浮岩相似,形成年代为17.7±2.1 ka。