最近130ka来黄土物质化学风化强度的空间格局

Chemical weathering intensity of loess deposits is largely determined by three factors: chemical weathering in source regions, grain size and post-depositional weathering. The third factor is influenced by climatic conditions such as precipitation and temperature, and the dust sedimentation rate in the area of deposition. Previous studies have shown that the (CaO+MgO+Na2O)/TiO2 ratio of decarbonated residue from loess is independent of grain size changes and thus is a reliable proxy for chemical weathering. However, the validity of (CaO+MgO+Na2O)/TiO2 to describe changes in monsoon intensity requires further study. In this study, 48 sections over the last glacial-interglacial cycle on the Chinese Loess Plateau were sampled, and the major elemental concentrations of 248 decarbonated residue samples were measured to investigate the utility of the (CaO+MgO+Na2O)/TiO2 ratio as a proxy for changes in monsoon intensity. Results show that the (CaO+MgO+Na2O)/TiO2 ratio, is relatively more sensitive to climate change than other indexes independent of grain size, and is not affected substantially by sedimentation rate. Assuming the weathering regime is relatively stable in the loess source regions, the (CaO+MgO+Na2O)/TiO2 ratio is a reliable proxy for the intensity of summer monsoon. A decreasing (CaO+MgO+Na2O)/TiO2 ratio from northwest to southeast both in loess and paleosols indicates that the Chinese Loess Plateau is in the control of the East Asian summer monsoon during both interglacial and glacial times. In addition, the spatial distributions of (CaO+MgO+Na2O)/TiO2 ratios show a greater north-south gradient during interglacial periods than during glacial periods. This may suggest that the spatial precipitation gradient, controlled by the summer monsoon, is steeper during interglacials than in glacials.

华北克拉通新生代玄武岩携带的地幔橄榄岩Fe同位素地球化学及含金云母地幔岩岩石学研究

In recent years, thanks to the improvement of analytical methods and the use of MC-ICP-MS, Fe isotope can be measured precisely. Fe isotope shows considerable variation both in biological and inorganic processes (from low T to high T) in nature, Therefore, Fe isotope has become one of the exciting frontier sciences and has favorable prospects of the application to the geosciences and life sciences. Based on a comprehensive review of available references in the related field, this study focuses on the development of techniques for high-precision measurement of iron isotope using MC-ICP-MS, and application of the techniques developed to study the Fe isotopes as well as major and trace element compositions of minerals (Ol, Opx, Cpx and Sp) from spinel peridotitic xenoliths from Cenozoic alkaline basalts to investigate Fe isotopic features of the lithospheric mantle beneath the North China Craton. The minerals from these xenoliths are similar to those off-cratonic peridotites world-wide, but are remarkably different from those on-cratonic peridotites and clinopyroxenes from these spinel lherzolites exhibit two types of chondrite-normalized REE patterns i.e. LREE-depleted and flat or spoon-shaped. It is noted that total abundances of REE in clinopyroxenes from these peridotites show a broad negative correlation with Cr# numbers of Cpx and Sp. The Fe isotope results show that the spinel peridotitic xenoliths have small but distinguishable Fe isotopic variations in minerals (generally Ol < Opx < Cpx) and samples, and the isotopic range in spinel is relatively large. Positive linear relationship with the ε57Fecpx/ε57Feopx ratio close to one unit has been observed between Fe isotopes of coexistent Opx and Cpx, indicating that the Cpx and Opx have generally reached Fe isotopic equilibrium. However, Fe isotopes between the Ol and Sp show apparent disequilibrium. The broadly negative correlation between mineral Fe isotopes and oxygen fugacity (fo2), metasomatic indexes such as spinel Cr#, (La/Yb) N and (La/Sm) N ratios of clinopyroxenes suggest that Fe isotopic variations in different minerals and peridotites were probably produced by melt-peridotite interaction. This study further confirms the previous observation that the lithospheric mantle has distinguishable and heterogeneous Fe isotopic variations at a scale of xenoliths. Mantle metasomatism that induces the interaction of the lithospheric mantle peridotite with metasomatic agent is a most potential mechanism for the Fe isotope fractionation in mantle peridotites. Therefore, Fe isotope could be a new and powerful tool to probe the evolution of the lithospheric mantle. We also report mineral compositions, clinopyroxene trace element concentrations and Sr-Nd isotopes for newly-discovered phlogopite-bearing spinel lherzolite and olivine clinopyroxenite xenoliths from three different localities （Hannuoba, Hebei Province; Jining Sangyitang, Inner Mongolia; Hebi, Henan Province）of the North China Craton. Systematic comparisons with phlogopite-free spinel lherzolite xenolith from the same locality reveals that the phlogopite-bearing peridotitic xenoliths have relatively higher Al2O3, CaO, Na2O, K2O, TiO2 contents and lower MgO contents than those phogopite-free counterparts. The former also has higher LREE concentrations, but relatively less radiogenic Sr-Nd isotopic ratios. This demonstrates that mantle metasomatism can not only enrich the basaltic components and trace element concentrations, but also make a decrease in Mg# of the peridotites and olivines and a relative depletion in Sr-Nd isotopes. 87Rb/86Sr-87Sr/86Sr isochrons of the phlogopite-bearing xenoliths indicate that mantle metasomatism happened in the Mesozoic and/or Cenozoic time. The metasomatic agent was derived from the asthenosphere. The result also manifests that the widespread similarity of the geochemical features such as major and trace elements and isotopic compositions in the Cenozoic lithospheric mantle beneath the North China Craton to those “oceanic” lithospheric mantle could be as a result of the ubiquitous presence of the interaction between the old refractory peridotites and the infiltrated asthenospheric melt, rather than the actually newly-accreted lithospheric mantle.

北大别黄土岭麻粒岩变质演化：对大别造山带演化进程的指示意义

Granulites from Huangtuling in the North Dabie metamorphic core complex in eastern China preserve rare mineralogical and mineral chemical evidence for multistage metamorphism related to Paleoproterozoic metamorphic processes, Triassic continental subduction-collision and Cretaceous collapse of the Dabieshan Orogen. Six stages of metamorphism are established, based on detailed mineralogical and petrological studies: (I) amphibolite facies (6.3–7.0 kbar, 520–550 °C); (II) high-pressure/high-temperature granulite facies (12–15.5 kbar, 920–980 °C); (III) cooling and decompression (4.8–6.0 kbar, 630-700 °C); (IV) medium-pressure granulite facies (7.7–9.0 kbar, 690–790 °C); (V) low-pressure/high-temperature granulite facies (4.0–4.7 kbar, 860–920 °C); (VI) retrograde greenschist facies overprint (1–2 kbar, 340–370 °C). The P–T history derived in this study and existing geochronological data indicate that the Huangtuling granulite records two cycles of orogenic crustal thickening events. The earlier three stages of metamorphism define a clockwise P–T path, implying crustal thickening and thinning events, possibly related to the assembly and breakup of the Columbia Supercontinent ca. 2000 Ma. Stage IV metamorphism indicates another crustal thickening event, which is attributed to the Triassic subduction/collision between the Yangtze and Sino–Korean Cratons. The dry lower crustal granulite persisted metastable during the Triassic subduction/collision due to lack of hydrous fluid and deformation. Stage V metamorphism records the Cretaceous collapse of the Dabieshan Orogen，possibly due to asthenosphere upwelling or removal of the lithospheric mantle resulting in heating of the granulite and partial melting of the North Dabie metamorphic core complex. Comparison of the Huangtuling granulite in North Dabie and the high-pressure (HP)–ultrahigh-pressure (UHP) metamorphic rocks in South Dabie indicates that the subducted upper (South Dabie) and lower (North Dabie) continental crusts underwent contrasting tectonometamorphic evolution during continental subduction–collision and orogenic collapse. High-pressure granulites are generally characterized by the absence of orthopyroxene. However, the Huangtuling felsic granulite rarely preserves the high-pressure granulite facies assemblage of garnet + orthopyroxene + biotite + plagioclase + K-feldspar + quartz. To investigate the effects of bulk rock composition on the stability of orthopyroxene-bearing, high-pressure granulite facies assemblages in the NCKFMASHTO (Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3) system, we constructed a series of P–T–X pseudosections based on the melt-reintegrated composition of the Huangtuling felsic high-pressure granulite. Our calculations demonstrate that the orthopyroxene-bearing, high-pressure granulite facies assemblages are restricted to low XAl [Al2O3/(Na2O + CaO + K2O + FeO + MgO + Al2O3) < 0.35, mole proportion] or high XMg [MgO/(MgO + FeO) > 0.85] felsic–metapelitic rock types. Our study also reveals that the XAl values in the residual felsic–metapelitic, high-pressure granulites could be significantly reduced by a high proportion of melt loss. We suggest that orthopyroxene-bearing high-pressure granulites occur in residual overthickened crustal basement under continental subduction–collision zones and arc–continent collision belts.

中国北方中新世风尘物质来源及古气候的地球化学指示

The most prominent tectonic and environmental events during the Cenozoic in Asia are the uplift of the Himalaya-Tibetan plateau, aridification in the Asian interior, and onset of the Asian monsoons. These caused more humid conditions in southeastern China and the formation of inland deserts in northwestern China. The 22 Ma eolian deposits in northern China provide an excellent terrestrial record relative to the above environmental events. Up to date, many studies have focused on the geochemical characters of the late Mio-Pleistocene eolian deposits, however, the geochemical characteristics of the Miocene loess and soils is still much less known. In this study, the elemental and Sr-Nd isotopic compositions of the eolian deposits from the Qinan (from 22.0 to 6.2 Ma) and the Xifeng (from 3.5 Ma until now) loess-soil sections were analyzed to examine the grain size effects on the element concentrations and the implications about the dust origin and climate. The main results are as follows: 1. The contents of Si, Na, Zr and Sr are higher in the coarser fractions while Ti and Nb have the highest contents in the 2-8 μm fractions. Al, Fe, Mg, K, Mn, Rb, Cu, Ga, Zn, V, Cr, Ni, LOI have clear relationships with grain-size, more abundant in the fine fraction while non significant relationship is observed for Y. Based on these features, we suggest that K2O/Al2O3 ratio can be used to address the dust provenance, and that VR (Vogt ratio = (Al2O3+K2O)/(MgO+CaO+Na2O)) can be used as a chemical weathering proxy for the Miocene eolian deposits because of their relative independence on the grain size. Meanwhile, SiO2/Al2O3 molar ratio is a best geochemical indicator of original eolian grain size, as suggested in earlier studies. 2. Analyses on the Sr and Nd isotope composition of the last glacial loess samples (L1) and comparison with the data from the deserts in northern China suggest that that Taklimakan desert is unlikely to be the main source region of the eolian dust. In contrast, these data suggest greater contributions of the Tengger, Badain Jaran and Qaidam deserts to the eolian dust during the last glacial cycle. Since the geochemical compositions (major, trace, REE and Sr, Nd isotope) of loess samples for the past 22 Ma are broadly similar with the samples from L1, these data trend to suggest relatively stable and insignificant changes of dust sources over the past 22 Ma. 3. Chemical weathering is stronger for Miocene paleosol samples than for the Plio-Pleistocene ones, showing warmer/more humid climatic conditions with a stronger summer monsoon in the Miocene. However, chemical weathering is typical of Ca-Na removal stage, suggesting a climate range from semiarid to subhumid conditions. These support the notion about the formation of a semi-arid to semi-humid monsoonal regime by the early Miocene, as is consistent with earlier studies.

新疆谢米斯台中段地区火山岩特征及其含矿性研究

Central Xiemisitai is located on the northwest edge of the Junggar Basin, bounded on the north by Sawuer Mountain, and southward Junggar Basin. Geotectonically, it is within the Chengjisi-Ximisitai-Santanghu island arc of Late Paleozoic, between Siberian and Junggar plates. The volcanics in this area mainly consist of acidic volcanic lava, rock assemblage of esite, dacite, and rhyolite, and the transitional phase is comparatively developed. Besides, Si2O of volcanics here covers a large range of 53.91-79.28t %, K2O of 1.71-6.94t%, and Na2O of 2.29-5.45t%, which is a set of metaluminous- peraluminous high K calc-alkaline to calc alkaline mid-acidic volcanic series. In addition, the volcanics are potassic to high-potassic assemblage, with slight shoshonite in. The REE curve of volcanics in central Xiemisitai is rightward and smooth, inclining to LREE enrichment, which reveals the characteristics of island-arc volcanics. Through the lithology changing from neutral to acidic, the negative anomaly of Eu is increasing. The volcanics here deplete HFSE such as Nb, Ti, P, etc., but relatively rich in LILE like Rb, K, Th, etc., possessing geochemistry characteristics of arc volcanics, which means that the lava source region is watery, under the meta-somatic contamination of subducted components. Moreover, high Ba and Sr show volcanics in epicontinental arc environment, and their contemporaneous granitoid rocks are also marked with the characteristics of volcanic arc granite. In central Xiemisitai, the volcanics zircon age of volcanic rhyolite is 422.5Ma± 1.9Ma, mid-late Silurian. Only one sample zircon has been measured for the present, not very convincing, so volcanics here might not come from Devonian volcanism. Consequently, further confirming the volcanic age will play a key role in the research on the beginning of volcanism in Xiemisitai area and even North Xinjiang. This area includes three copper mineralization types: a) from andesite fracture; b) from rhyolite fracture broken zone, with the copper mineralization distributed by veins along the fissure; and c) from quartz veins. The mineralization of earth surface in S24 ore spot is intensive, and the primary geochemistry reconnaissance anomaly is fairly good. According to display data, the maximum content of Cu is as high as 0.9% and as low as 0.05%. Also, ore-control fracture structure is having a considerable scale in the strike of fracture both horizontally and vertically downwards, and the result of the geophysics stratagem EH-4 system reveals obvious low-resistivity anomaly. As a result, we believe that the S24 plot is expected to be a volcanic copper deposit target area.

泥河湾盆地大长梁剖面岩石磁学和环境磁学研究

The fluvio-lacustrine sequence in the Nihewan Basin is an important archive of late Pliocene-Pleistocene climate and environment changes in temperate northern China, which provides excellent sources of early human settlements in high latitude East Asia. The recent years have witnessed a considerable progress in the paleomagnetic dating of its stratigraphy, which has notably increased our understanding of a series of important issues such as the early human occupation in the Old World, the infilling history of the Nihewan Basin, and the chronological sequence of the Nihewan faunas. Up to now, the long-term paleoenvironmental changes directly retrieved from this basin, which might influence the evolution and expansion of early humans in the Nihewan Basin, are still poorly constrained, although several paleoclimatic records have been retrieved from this area. In this study, a combined mineral-magnetic and geochemical investigation was carried out on the fluvio-lacustrine sequence from the Dachangliang section at the eastern margin of the basin in order to reveal its rock magnetic and environmental magnetic characteristics and its implications for early human evolution in East Asia. The major findings and conclusions are listed as the following: First, there is an increased cooling coupled with an intensified aridification recorded in the fluvio-lacustrine sequence of the Dachangliang section. The cooling is related to an up-section decrease in propensity to chemical weathering as inferred from an increase in low-field susceptibility after cycling to 700 °C. Close to 700 °C, reacting chlorite is providing the iron source for newly formed very fine-grained ferrimagnetic minerals which enhances the susceptibility signal. The reactivity of chlorite after annealing at temperatures above 600 °C is documented with X-ray diffraction. Second, degrees of chemical weathering in the Nihewan Basin are further estimated by clay mineralogy (i.e. chlorite and illite contents and chlorite/illite ratio) and a series of major element proxies (i.e. Na2O/Al2O3 versus K2O/Al2O3 diagram, Al2O3-(CaO + Na2O)-K2O ternary diagram (A-CN-K), chemical index of alteration (CIA), (CaO + Na2O + MgO)/TiO2, (CaO + Na2O + MgO + K2O)/(TiO2 + Al2O3), CaO/Al2O3 and CaO/TiO2). The up-section decrease in propensity to chemical weathering suggested by the aforementioned rock mangetic measurement is further confirmed by these geochemical analyses. Combining the chemical weathering records from the Nihewan Basin, Chinese Loess Plateau, South China Sea and eastern China, we find that the consecutive decreasing trend in chemical weathering intensity during the late Cenozoic is ubiquitous across China. This pattern may result from a long-term decreasing East Asian summer monsoon and increasing East Asian winter monsoon, and thus a consecutive increasing of aridification and cooling in Asia during the Quaternary. Furthermore, the chemical weathering intensity increased from South China to North China during the Quaternary, in line with the decreasing East Asian summer monsoon and increasing East Asian winter monsoon and thus the gradually intensified aridification and cooling from South China to North China. Third, a combined mineral-magnetic and geochemical investigation provides evidences that the large-amplitude alterations of concentration of magnetic minerals mainly result from preservation/dissolution cycles of detrital magnetic minerals in alternately oxic and anoxic depositional environments. The preservation/dissolution model implies that the high-magnetic and low-magnetic cycles of this sedimentary sequence represent glacial and interglacial climate cycles, respectively. This contribute significnatly to our understanding of the link between climate and magnetic properties. Finally, the paleoclimatic implications of these rock magnetic and geochemical characteristics significantly increase our understanding of the general setting of early humans in high northern latitude in East Asia. We propose that the cold and dry climate may have contributed significantly to the expansion and adaptation of early humans, rather than bringing hardship, as is often thought. The relationship between magnetic properties and climate possibly provides valuable information on the climatic context of the Paleolithic sites in the basin, especially whether the occupation occurred during an interglacial or glacial period.

特提斯喜马拉雅带晚白垩世-始新世沉积源区分析及其构造意义

Tethyan Himalayan Sequence (THS) is located at the frontier of the India-Asia collision zone, which can preserve critical information about collision. This paper reports detailed petrology, geochemistry, spinels electron microprobe data, and in situ U-Pb ages and Lu-Hf isotopic data on detrital zircons from the late Cretaceous to early Eocene strata in Gyantze and Gamba area, south Tibet that provide important constraints on the early tectonic evolution of the India-Asia collision. In Gyantze, the lithic arkose in Zongzhuo mélange is characterized by, SiO2 =80.4%, Al2O3=8.6%, Na2O=1.6%, K2O=1.1%, LaN/YbN=8.90, and εNd (0) =-10.27. Spinels compositions are characterized by low TiO2 (generally <0.1%) and a Cr number mainly between 70 and 80. The largest population of detrital zircons is within the 73-169Ma range with high εHf (t) and > 500 Ma with complex εHf (t) values. The lithic arkose in Rilang conglomerate is characterized by, SiO2 =56.5%, Al2O3=15.6%, Na2O=4.7%, K2O=0.6%, LaN/YbN=5.00-5.29, and εNd (0) =1.92. Spinels of 2006T98 display high TiO2 (generally >0.2%) and a Cr number mainly between 70 and 85, other spinels are characterized by low TiO2 (generally <0.2%) and a Cr number mainly between 60 and 90. The largest population of detrital zircons is within 90-146 Ma range with high εHf (t). The lithic arkose in Jiachala formation is characterized by, SiO2 =64.6%, Al2O3=12.1%, Na2O=1.9%, K2O=1.8%, LaN/YbN=7.73-9.13, and εNd (0) =-5.52~-8.43. Spinels in the Jiachala formation have low TiO2 (generally <0.2%) and a Cr number between 39 and 88. Detrital zircons have a wide range of age distribution of 82-3165Ma with complex εHf (t). In Gamba, The quartze sandstone in Jidula formation is characterized by, SiO2=97.4%, Al2O3=0.9%, Na2O=0.03%, K2O=0.18%, LaN/YbN=18.70-21.684, and εNd (0) between -13.1~-7.4. While the lithic arkose in Zhepure formation is characterized by, SiO2=68.4%, Al2O3=7.3%, Na2O=1.15%, K2O=0.52%, LaN/YbN=6.09-8.99, and εNd(0)=-5.8~-6.3. Based on our geochemical analysis, spinles electron microprobe data, U–Pb ages and Hf isotope data for detrital zircons of the late Cretaceous-Eocene strata in Gyantze and Gamba, southern Tibet, the following major conclusions can be drawn: 1. In Gyantze, the Zongzhuo mélange was mainly derived from accretionary prism/THS of continental slop and Gangdese arc. Rilang conglomerate was totally from Gangdese arc. The Jiachala formation was derived from THS, suture zone and Gangdese arc. 2. In Gamba, the Jidula formation was from India craton, while the Zhepure formation was derived from THS, suture zone and Gangdese arc. 3. The deposite of Zongzhuo mélange and Rilang conglomerate (73-55Ma) marks the collision between India and Asia. 4. Late Paleocene-Eocene tectonic evolution is consistent with foreland basin system.

广西凭祥英安岩的化学风化作用研究

利用薄片观察、X射线粉晶衍射分析和化学分析方法研究了广西凭祥英安岩风化剖面的形成作用。风化作用初期,母岩中微量黄铁矿的氧化分解导致方解石与绿泥石的迅速分解;风化中期形成了大量的高岭石、伊利石、蒙脱石和蛭石;风化作用高级阶段以高岭石、石英和氧化铁矿物的富集为特征,但仍然存在少量蒙脱石、伊利石和蛭石。风化剖面的部分层段显示出与剖面其他部分明显不同的地球化学特征,即Na的富集和K的亏损。在Al2O3-(CaO+Na2O)-K2O三角图上,风化中期这些层段明显偏离了正常的风化趋势。矿物学和微形貌的研究表明,造成偏离的原因是古地下水引起的正长石的钠长石化作用。

流体—岩石相互作用—流体包裹体及蚀变岩某些成分变异的特征

以五龙金矿为例，在同一剖面上采集了不同成矿阶段的含金石英脉和近矿蚀变岩样品，．根据矿物流体包裹体和蚀变岩成分地的测定结果，系统地讨论了不同成矿阶段流体包裹体成（K^+、Na^+、Ca^＋、CO2等）和不同蚀变岩成分（K2O、Na2O、CaO1、MgO、CO2等）的变化规律，结果表明，从第二成矿阶段到第四成矿阶段，流体中的Na^+、Ca^+、Mg^+含量有明显的升高的趋势，K^+ 含量降低，蚀变岩中，K2O由远离矿体的原岩到靠近矿体的蚀变岩其含量增加，而Na2O、CaO、MgO逐渐降低，这种流体包裹体和蚀变岩中的某些相应成分的变化特征，应该是流体－岩石相互作用的结果。

贵州老万场金矿床红土化作用及对金赋存状态的制约

红土的矿物成分、硅酸盐成分和化学蚀变指数(CIA)特征表明，贵州老万场红土化过程经历了粘土化、铝土矿化、铁化三种化学风化作用；地球化学上属早期红土化作用阶段。风化过程中K2O、Na2O、CaO相对于Al2O3的淋失量很高，显示较强的化学蚀变作用。红土剖面中成矿元素Au、As、Sb含量变化很大。Au通常为40～4551ng／g；Sb普遍小于0．1％；As通常为0．n％，最高1．8％。它们普遍高于大厂层硅质岩中相应成矿元素的含量。红土化过程中，Au、As的富集与Fe的富集作用有明显的关系，并与红土剖面中相对还原的部位有关，而sb则在相对氧化的环境中聚集。红土剖面中稀土元素含量普遍较高，轻重稀土元素之比通常小于10，稀土元素分布模式非常相似，与大厂层稀土元素特征有明显差别。微量元素图解显示红土剖面中的大离子亲石元素、不相容元素与大厂层也有明显的不同。成矿元素问的差异性也很明显，表明大厂层岩石可能并不为老万场红土层的发育提供直接的物质来源，红土层的发育似乎与后期第四系的冲积物、洪积物有关。成矿物质也可能不与大厂层直接有关。带能谱的透射电镜(TEM-EDX)观察表明金的存在形式多样，虽然金主要以浑圆状的自然金颗粒产出，但也可以被针铁矿(含砷针铁矿)、石英硅质吸附，或被伊利石等吸附。显示红土化过程中铁还原作用对金富集的重要性。

贵州老万场金矿成矿物质来源的研究

硅酸盐成分研究表明老万场红土化过程经历了粘土化、铝土矿化、铁化三种化学风化作用;风化中化学蚀变较强,K2O、Na2O、CaO 相对于Al2O3 的淋失量很高。在红土剖面上成矿元素的含量变化很大。相关性分析表明,红土化过程中,Au、As 的富集与脱硅富铝化程度关系不大,而与铁的富集有明显的关系;Au、As 的富集与相对还原的环境有关,而Sb 则在相对氧化的环境中易于聚集。可能反映了红土剖面中潜水面下部附近相对还原的环境Au、As 易于富集。大厂层样品和老万场红土剖面样品在成矿元素(Au、As、Sb) 含量、稀土元素含量、轻重稀土比值、配分曲线上以及微量元素特征包括大离子亲石元素、不相容元素上的较大差异,显示了红土层的发育与大厂层岩石有较大的差别,暗示它们物质来源上的一定差异性。

胶东北部碱性超基性脉岩地球化学特征及环境和成因探讨

胶东地区脉岩属碱性超基性岩系(Na2O+K2O=4．67％～5．43％；SiO2=36．70％-39．99％)，岩性为单一的橄榄辉石岩。从主量元素(包括CIPW标准矿物组成)和过渡元素组成来看，该岩系近似原始岩浆组成。电子探针结果显示：橄榄石为富镁质橄榄石(贵橄榄石)(Fo=71—90)，单斜辉石为透辉石(次透辉石为主)。岩石富集大离子亲石元素(K、Rb、Sr、Th和Ba)，但不具有高场强元素(Nb、Ta、Zr和Hf)的亏损，表明岩石形成于大陆板内环境。为地幔橄榄岩低度部分熔融(3．4％)的产物。同时，它具有大陆边缘弧的特性。暗示其为一种滞后型弧岩浆作用的产物。稀土元素特征显示，岩石强烈富集LREE，而相对亏损HREE，暗示了源区的富集特性。Eu／Eu^+=0．89—1．00，总体不表现明显的负Eu异常，暗示斜长石不是主要的分馏矿物相。结合板内碱性岩石的矿物结晶顺序认为，本区岩浆分馏以较弱的橄榄石分馏为主。

湘东北构造活化期花岗岩形成构造环境及成因

华南大陆从印支期进入陆内活化阶段，形成一系列沿断裂分布的印支期花岗岩。该期花岗岩代表陆内活化期的开发，在岩石地球化学和形成构造环境上以及岩石的形成方式上都与该期以后的花岗质岩石存在一定的差异。表明华南中生代陆内活化构造环境的复杂性和多阶段性。本文以湘东北印支期花岗质侵入岩为例，分析其成岩构造环境，用以反映该区陆内开始活化时的构造环境和活化方式。湘东北印支期花岗质岩石以花岗闪长岩－二长花岗岩岩石系列为主，以相对较低的SiO2、K2O和较高的TiO2、P2O5及负铕异常不明显区别于中生代其它侵入期次岩石，具有陆壳重熔型花岗岩特征，其形成为陆内俯冲导致陆壳物质在俯冲前缘重熔的结果，表明陆内俯冲作用是导致湘东北陆内开始活化的主要方式之一，其构造环境以逆冲式推覆挤压为主。

湘东北中生代基性岩脉岩石地球化学特征

湘东北基性岩脉以K2O/Na2O<1,K2O+Na2O在3.03-5.57之间为特征。属陆内拉斑玄武岩系列，部分煌斑岩属于碱性系列，岩石富LREE，δEu负异常不明显，其形成主要受软流圈地幔部分熔融作用制约，岩石微量元素总体上具有富集地幔（EMⅡ型）洋岛玄武岩（OIB）特征，富集Nd,P,Cs，b,Sr,U,Th等富集程度不明显，Ｔa,Nb略有富集，湘东北的北部地区基性岩脉虽表现出Ｔa,Nb,Ti亏损，但ＬＩＬＥ并不富集，可能反映地壳混染程度的增强，基性岩脉的特质组成与湘东南玄武岩类岩石基本一致，推测两者应具有同一个地幔源区，依据侵入关系和区域对比，认为湘东北基性岩脉的形成是早白垩世以后晚燕山期岩石圈拉张－减薄作用产物，湘东北基性岩脉所处的构造环境应属于整个湘东南岩石圈伸展－减薄带的一部分。

湖南板溪脉岩的年代学、岩石学、地球化学及其构造环境

湘中地区的锑（金）成矿多与脉岩相伴生，对其成因及对矿床成矿作用的贡献至今仍不清楚。本文通过对板溪石英斑岩脉的研究表明，全岩K，Ar年龄为200Ma±，至少比板溪锑矿成矿晚200Ma，因此脉岩与成矿不存在成因联系。板溪脉岩显示过铝质特征，其A／CNK为1．08～2．61，A／NK为1．08～2．71，K20为0．9％～3．66％，K20±Na2O为4．28％～7．5％，大多数样品的K2O／Na2O小于1。稀土元素以相对富集LREE，LREE分馏不明显、HREE分馏明显，配分曲线右倾为特点，其（LJYb）n为14．47～28．11，（Gd／Yb）n为9．8～14．3，并且具强负Eu异常，δEu为0．05～0．14，这表明岩石在成岩演化过程中，经历了强的分异结晶作用。在原始地幔标准化蛛网图上，脉体相对富集LILE，并具Ti和P负异常。（^87Sr／^86Sr）i变化范围较大，为0．6653～0．7149，其中B-3样品的（^87Sr／^86Sr）i为0．7149，与来自地壳上部的花岗岩的^87Sr／^86Sr初始比值相当。在构造环境判别图中，样品显示岛弧／同碰撞花岗岩的特点。结合前人的研究成果，笔者推测板溪脉岩为地壳上部岩石部分熔融的产物，并经历了混染和高度的分异演化。其动力来源为200Ma左右，地壳叠置加厚作用后幔源岩浆的底侵作用.