The Geochemical Features and Evolution of Laterite in the Karst Areas of Guizhou Province

Guizhou is one of the dominating karst regions,where laterite is widespread,in China.Seventy-two laterite samples were taken from twelve laterite sections in the karst areas of Guizhou Province,which are possessed of typical subtropical karst geomphological and ecological environmental features and have evolved completely from dolostones and limestones.In terms of the major,trace and REE deta for thd samples this paper discusses the geochemical characteristics of laterite in the karst areas with an attempt to disclose the geochemical process and evolutionary rule of laterite formation.There have been involved three important pedogenetic geochemical precesses in the formation of laterite in the karst areas:(1)enrichment of silicon and aluminum and depletion of calcium and magnesium;(2)enrichment of iron and manganese;and(3) enrichment of aluminum and depletion of silicon.During the formation and evolution of laterite,obvious enrichment and differentiation of trace elements such as Cu,Pb,Zn,Ni,Co,Cr,Pb,F,Cl,and As can be observed,but for the rare-earth elements,their enrichment is remarkable against a weak differentiation.The REE distribution patterns in the laterite are similar to those of its parental carbonate rocks.     

Geochemistry of sedimentary rocks and its relation to crustal evolution and mineralization in Southwest Yangtze Massif, China

Through a systematic study on trace elements and REE geochemistry of mudstone deposited in the basin and lower slope environments during Upper Proterozoic to Triassic in the Southwest Yangtze Mssif,three geochemical abnormal horizons of which the geochemical characteristics are quite different from those of other horizons have been established for the first time.They are the Lower Cambrian,the Upper Devonian and the Upper Permian,As compared with the crustal evolution in this area.these three geochemical abnormal horizons are corresponding to the pulling-apart periods of geotectonic cycles.which illustrates that uncommon depositional sources puring into the basin from the earth‘s interior may be one of the most important causes to originate the geochemical anomalies in these lhrizons.Thus it can be realized that the geochemistry of post-Archean sedimentary rocks has a great deal to do with the crustal evolution and it can be used as a tracer to analyze the crustal evolution.The elements in this area are mainly concentrated in these geochemical abnormal horizons,and the degree of enrichment and deficiency of trace elements in other horizons is very limited.A series of research on mineralization indicates that the main strata-bound ore deposits discovered in the Southwest Yangtze Massif occur in the Cambrian,Devonian and Permian-Trassic strata.The results of isotope tracer resarch have also proved that most of the metallogenic elements in these ore deposits came from the host strata.which illustrates that the geochemical abnormal horizons may have made great contributions to these ore-forming processes.Thus it can be concluded that it is only the particular horizons corresponding to the particular periode of earth‘s evolution that can they be the significant source beds because only in these uncommon horizons there can be highly enriched metallogeinc elements.which may be one of the most important reasons for explaining the time-bound nature of mineralization.     

Stratigraphic Division and Correlation of the Nihewan Beds by Multivariate Statistical Analysis

Described in paper is the principle of optimal partitioning method for stratigraphic division and correlation.The Nihewan Beds are taken for example to show how to apply this approach in stratigraphic division and correlation.The semiquantitative spectral analysis data on aggregate trace elements in 324 samples taken from the nine sections in the Nihewan Basin are treated with multivariate statistical method for stratigraphic division and correlation.First ,the data from all the sections are respectively calculated by the optimal partitioning method to establish the stratigraphic boundaries.The optimal partitioning method has proved itself to be applicable to stratigraphic division and correlation. In our practice the Nihewan Beds are divided into five zones (I-V).Zone I includes subzones Ia and Ib,Zones Ia,Ib,II and III are considered to be corresponding to the Pliocene(N2),the early Early Pleistocene,the late Early Pleistocene,and the Middle Pleistocene,respectively .Zones IV and V are probably Late Pleistocene in age.This indicated that sediments deposited con-temporaneous in the sections of the same basin are similar in geochemical characteristics,although dif-ferent in geographical location.However,the sediments also show some variations ,with a transitional relationship from one section to another .For example ,in Zone II,the sediments of the Xiaodukou section show not only the characteristics of the Nangou-Hongya and Hutouliang sections,but also those of the Xiashagou,Shixiaxi,Shixiadong and Wulitai sections.It can be seen from the above that the zones can be characteristically correlated with one another.In addition the feasibility of the optimal partitioning method is also described in the present paper.     

Petrogenesis and Geochemistry of Neogene Basalts in Eastern Anhui

The basalt terrain of the Neogene Huangguoshan and. Guiwu Formations of eastern Anhui on the east side of the Tancheng-Lujiang fault belt is one of a few Cenozoic basalt terrains in eastern China for which detailed geochemical study has not been conducted. This paper reports the abundances of major elements and more than 20 trace elements (including REE) of 22 samples and the Nd, Sr and Pb isotopic compositions of 11 samples from the eastern Anhui basalt terrain, thus more or less systematically revealing the geochemical characteristics of this continental basalt suite. The paper discusses the origin of the basalt suite and the character and process of its mantle source. The basalt suite was derived from a heterogeneous continental lithospheric mantle with end members characteristic of the EMI-type oceanic basalt mantle, which was affected by mantle metasomatism (or enrichment of trace elements) and was characterized by a multi-stage evolution under open conditions.     

Geochemical Characteristics of the Sandstone—type Uranium Deposits in Northern Sichuan Province

Described in detail in this paper are the geochemical characteristics of the sandstone-type uranium deposits in northern Sichuan Province .Favorable and unfavor-able conditions for the enrichment of uranium are explored on the basis of the elemental abundances and ratios of U,Th and K determined by gamma-ray spectroscopy. Gamma-ray spectroscopic(U, Th and K) and XRF analyses(As, Ba) are helpful to distinguish U-bearing light-colored sandstones from U-barren light-colored ones and red sandstones (red claystone) from light-colored sandstones. Therefore, the favorable target area for prospecting the sandstone-type uranium deposits can be difined in northern Sichuan Province.     

Two Types of Granites in the Western Yangtze Block and Their Implications for Regional Tectonic Evolution: Constraints from Geochemistry and Isotopic Data

In the western Yangtze Block, widespread Mesoproterozoic to Neoproterozoic rocks are the key to understanding the Precambrian tectonic-magmatic evolution of the region. However, their petrogenesis and tectonic setting are still controversial. In this paper, zircon U-Pb ages, Sm-Nd isotopic and whole-rock geochemical data are reported from selected fresh samples in the southern Dechang county, southwestern China, in order to constrain their emplacement age and magma source, as well as their petrogenesis and tectonic setting. They are mainly composed of biotite monzogranite, monzonitic granite, biotite granodiorites, and quartz diorite. Two ages of 1055 ± 43 Ma and 837.6 ± 3.8 Ma were obtained through zircon U-Pb dating by LA-ICP-MS and LA-MC-ICP-MS, respectively. According to their major element compositions, the Grenville-age granites are peraluminous calc-alkaline series calcic S-type granite. In contrast, the mid-Neoproterozoic granites are metaluminous calc-alkaline series alkalic I-type granite. Furthermore, the S-type granites are enriched in LREEs relative to HREEs with(La/Yb)_N ratios of 3.85–18.56 and underwent major fractionation with strongly negative Eu anomalies(Eu/Eu~* = 0.38–0.66). In the MORB-normalized trace element variation diagram, all the samples are enriched in Ce and large ion lithophile elements such as Rb, Th, and K, and depleted in high field strength elements such as Nb, and Ti, with negative Sr and Ti anomalies. The I-type granites are enriched in LREEs with slight negative Eu anomalies(Eu/Eu~* = 0.83–0.93). They are characterized by the enrichment of highly incompatible elements(such as K, Rb, Ba, Th) and LREEs, relative to MORB. Neodymium isotopic data show that the S-type granites display ~(143)Nd/(~(144) Nd) values of 0.51241–0.51256, and have ε_(Nd)(t = 1055 Ma) values of(-3.29) to(-3.81). Calculated t_(DM) ages yield values from 1.87 to 1.91 Ga with the t_(DM).2 stg ages of 1.86 to 1.9 Ga. The I-type granites have ~(143)Nd/(~(144) Nd) ratios between 0.51192 and 0.51195, corresponding to initial ε_(Nd)(t = 837 Ma) values of 1.22 to 5.63. Calculated t_(DM) ages yield values from 1.0 to 1.38 Ga and the t_(DM).2 stg ages yield values from 0.99 to 1.06 Ga. The S-type granites are distinguished as syn-collision granite, whereas the I-type granites were formed as arc magmas according to the Rb-(Yb+Ta) and R_1-R_2 tectonic discrimination diagrams. To conclude, there are two types of spatially associated granite, the Mesoproterozoic S-type granite which were derived from remelting of upper crustal mudstone and/or clastics and resulted from the convergence of two continental plates, and the mid-Neoproterozoic I-type granite which formed in continental arc and resulted from mantle-derived magma mixed crust material, in the western Yangtze Block. Furthermore, we suggest that collision between the Yangtze and Cathaysia blocks occurred at about 1055 Ma, and caused the Stype granite. The I-type granite related to the subduction of oceanic lithosphere eastward underneath the Yangtze Block in the mid-Neoproterozoic.     

Hydrothermal Zircon Geochronology in the Shangxu Gold Deposit and its Implication for the Early Cretaceous Orogenic Gold Mineralization in the Middle Bangonghu–Nujiang Suture Zone

As a typical orogenic gold deposit in Tibet, Shangxu gold deposit is located at the Bangong Lake–Nujiang River Metallogenic Belt in the south of Qinghai–Tibet Plateau. In this paper, zircon U-Pb dating, trace elements and Hf isotopic analysis were performed on Au-bearing quartz veins in the Shangxu gold deposit. Zircons from Au-bearing quartz veins can be divided into three types: detrital, magmatic, and hydrothermal zircons. There are two age peaks in detrital zircons: ca. 1700 Ma and ca. 2400 Ma. There are two groups of concordant ages including 157 ± 4 Ma(MSWD = 0.69) and 120 ± 1 Ma(MSWD = 0.19) in magmatic zircons, in which εH f(t) value of ca. 120 Ma from the magmatic zircons range from +8.24 to +12.9. An age of 119 ± 2 Ma(MSWD = 0.42) was yielded from hydrothermal zircons, and their εH f(t) values vary between +15.7 and +16.4. According to sericite Ar-Ar age, this paper suggests that an age of 119 ± 2 Ma from hydrothermal zircons represent the formation age of the Shangxu gold Deposit, and its mineralization should be related to the collision between Lhasa Block and Qiangtang Block. The metallogenic age is basically the same as the diagenetic age of Mugagangri granite, and εH f(t) value of hydrothermal zircon is significantly higher than that of the contemporaneous magmatic zircon, which indicates that there is a genetic relationship between the gold mineralization and the deep crust-mantle magmatism.     

Tectonic Setting and Nature of the Provenance of Sedimentary Rocks in Lanping Mesozoic- Cenozoic Basin： Evidence from Geochemistry of Sandstones

The geochemical composition of sandstones in the sedimentary basin is controlled mainly by the tectonic setting of the provenance, and it is therefore possible to reveal the tectonic setting of the provenance and the nature of source rocks in terms of the geochemical composition of sandstones. The major elements, rare-earth dements and trace elements of the Mesozoic-Cenozoic sandstones in the Lanping Basin are studied in this paper, revealing that the tectonic settings of the provenance for Mesozoic-Cenozoic sedimentary rocks in the Lanping Basin belong to a passive continental margin and a continental island arc. Combined with the data on sedimentary facies and palaeogeography, it is referred that the eastern part of the basin is located mainly at the tectonic setting of the passive continental margin before Mesozoic, whereas the western part may be represented by a continental island arc. This is compatible with the regional geology data. The protoliths of sedimentary rocks should be derived from the upper continental crust, and are composed mainly of felsic rocks, mixed with some andesitic rocks and old sediment components. Therefore, the Lanping Mesozoic-Cenozoic Basin is a typical continental-type basin. This provides strong geochemical evidence for the evolution of the paleo-Tethys and theb asin-range transition.     

The Source System of Mesozoic Magmatism in Middle and Lower Yangtze Valleys and Adjacent Areas—A Geochemical Tracer Study

The mechanism that controls regional mineralization must be governed by a series of geochemical reactions in relation to the source system of magmatism.In this respect,the geological bodies must have been overprinted by various kinds of tracers in terms of either chemical elements or isotopes.For this reason,the problem may be better approached by treating the lithosphere as a whole with due considerations given to the various tracer elements and isotopes and the various media of the magmatism(magmatic as well as sedimentary rocks).Presented in this paper are the results of this attempt based on a great wealth of available data concerning the source system of Mesozoic magmatism in the Middle and Lower Yangtze Valleys and adjacent areas.     

The Behaviour of Rare—Earth Elements（REE） During Weathering of Granites in Southern Guangxi,China

The behaviour of the rare-earth elements(REE)during the weathering of granites was studied in southern Guangxi,China.Based on the study of the weathering profiles,the soil,weathered and sub-weatereb zones are identified with different REE geochemical behaviours throug the weathering profiles of granite.The Ce anomalies of the weathering profiles cover a large range of values with most falling between 1.02 and 1.43in the soil zone and 0.16and 0.40in the weathered and sub-weathered zones.Light rare-earth elements(LREE) and heavy rare-earth elements(HREE)are enriched to varying degree in the weathering profiles as compared to host granites.In the soil zone,more HREEs are leached than LREEs,and HREEs are more enriched than LREE in the weathered and sub-weathered zones.It is considered that infiltration and adsorption on clays are two processes controlling the enrichment and formation of REE deposits in the weathering profiles of granite.     

Geochemistry and petrogenesis of Permian granitoids in the northwestern margin of the North China Craton: insights from the Dongshengmiao pluton,Inner Mongolia

Permian granitoid emplacement represents one of the most important tectonothermal events in the northern margin of the North China Craton (NCC). In this study, we collected geochronological and geochemical data of the regional Permian granitoid in the northwestern margin of the NCC, and investigated the Dongshengmiao pluton, using it as an example to constrain the regional granitoid petrogenesis and its geodynamic settings. The Dongshengmiao pluton contains porphyritic granite and quartz diorite. LA-ICP-MS zircon U-Pb dating results have constrained the granitoid emplacement to be ca. 287?275 Ma. The Dongshengmiao granitoids have a SiO₂ range of 58.4?76.5%, moderate to high alkali content (Na₂O + K₂O = 5.16–7.94%), and are rich in large-ion lithophile elements (LILEs; e.g. Rb, Ba) and depleted in high-field strength elements (HFSEs; e.g. Nb, Ta, Ti). The zircons in quartz diorite have ε_Hf(t) values of ?15.6 to ?11.1 with two-stage Hf model ages (T_DM2) of 1997–2281 Ma, suggesting that the magma was derived from partial melting of old continental materials. In contrast, porphyritic granite shows variable Hf isotopic composition with ε_Hf(t) values of ?13.7 to ?2.6 and T_DM2 of 1471–2167 Ma, indicating a heterogeneous magma source. Besides the Dongshengmiao pluton, all the Permian granitoids in the northwestern margin of the NCC exhibit similar geochemical characteristics, including enrichment in LILEs, depletion in Nb and Ta, and enriched Hf isotopic signatures. The comprehensive geochemical data indicate that these Permian granitoids are derived from magma mixing between dominant partial melting of ancient felsic crustal materials and minor juvenile basaltic magma. Tectonically, the Dongshengmiao and other granitoids in the northwestern margin of the NCC may have been formed in a post-collisional extensional setting.     

Neoproterozoic granitic gneiss offshore the Shandong Peninsula of Eastern China: the eastward extension of the Sulu Orogenic Belt

The Sulu Orogenic Belt in eastern China has experienced a multistage tectonic evolutionary history. However, its geological evolution has not yet been corroborated by sufficient direct evidence from basement rocks. Chaolian Island on the Qianliyan Uplift provides an opportunity to study the formation and evolution of the Sulu Orogenic Belt using direct geochronological and geochemical evidence. We determined that the characteristic mineral assemblage in the study region is quartz + K-feldspar + perthite + biotite + muscovite. The samples are silica- (SiO₂ = 72.8%–75.8%) and alkali-rich (ALK(Na₂O+K₂O) = 8.7%–9.3%), with high iron-magnesium ratios (FeO*/(FeO*+MgO) = 0.92–0.96) and low CaO and MgO concentrations. Furthermore, they are rich in large-ion lithophile elements K, Rb, Ba, and U, but depleted in high field strength elements Nb, Ta, and Zr. They exhibit high Ga/Al values (Ga × 10⁴/Al = 3.33–3.74) and significant fractionation between light and heavy rare earth elements. The samples are A-type granites. In the discrimination diagrams for granite genesis types, the samples plotted in the post-orogenic A2-type granite region. Secondary ion mass spectrometer (SIMS) zircon U–Pb dating results indicated that the granitic gneiss formed ~782.6–802.3 Ma (Middle Neoproterozoic), consistent with the timing of the breakup of the Rodinia supercontinent on the northeastern margin of the Yangtze Plate. Comparing geochemical characteristics and zircon U–Pb ages of the A-type granitic gneisses of the Sulu Orogenic Belt, the Qianliyan Uplift appears to be an extension of the belt across the ocean and is affiliated with the Yangtze Plate. The granitic gneiss on Chaolian Island is related to the formation of a mantle superplume during the breakup of Rodinia, and the northeastern margin of the Yangtze Plate during the Middle Neoproterozoic was located in a back-arc extension setting induced by the subduction of oceanic plates.     

长江中下游贵池矿集区燕山期岩浆作用及其地质意义:年代学、地球化学及Sr-Nd-Hf同位素证据

长江中下游贵池地区燕山期侵入岩发育,与成矿关系密切.本文对该区侵入岩开展了详细的锆石U-Pb年代学、地球化学及Sr-Nd-Hf同位素研究.结果表明,马头花岗闪长斑岩形成于147±2Ma,而花园巩石英二长岩形成于127±1Ma,比花岗闪长斑岩晚约20Ma.早期的花岗闪长斑岩(147 ～ 145Ma)为高钾钙碱性系列,具有高Al2O3、Sr含量和Sr/Y、La/Yb比值,以及低的Y、Yb含量,与埃达克质岩的地球化学特征一致;而晚期石英二长岩(127Ma)和石英正长岩为钾玄岩系列,具有高的(Na2O+ K2O)、Zr、Nb、Y含量和Y/Nb、Yb/Ta比值,与造山带A2型花岗岩地球化学特征相似;碱长花岗岩(125～124Ma)同样具有A型花岗岩的地球化学特征,但与石英二长岩、石英正长岩相比,碱长花岗岩的Y/Nb、Yb/Ta比值相对较低,具板内环境A1型花岗岩的地球化学特征.因此,贵池地区岩浆岩从早期的埃达克质岩变为晚期的A型花岗岩,反应了晚中生代时期长江中下游地区的构造环境由大陆边缘环境向伸展环境的转变.     

阿尔金造山带青白口纪片麻状花岗岩的厘定及对Rodinia超大陆汇聚时限的制约

阿尔金造山带新元古代花岗岩的研究对探讨该地区Rodinia超大陆汇聚阶段构造演化过程具有重要意义。本文对在亚干布阳一带新厘定的青白口纪片麻状花岗岩开展了详细的岩石学、年代学和岩石地球化学研究。锆石LA-ICP-MS U-Pb年代学证据显示片麻状花岗岩结晶年龄分别为(883.0±3.3)Ma和(883.1±3.3)Ma,说明其侵位于青白口纪。地球化学结果显示,常量元素具有富硅、铝、钾和低钠、镁、钙和钛的特点,具钙碱性-高钾钙碱性、过铝质花岗岩特征。岩石轻稀土分馏较强而重稀土分馏较弱,具有明显的负Eu异常,总体呈右倾的"V"型稀土分配模式。岩石富集Rb、Th、LREE等大离子亲石元素,中等亏损Ba,强烈亏损Nb、Sr、P、Hf、Ti等高场强元素,总体特征显示了典型的壳源花岗岩的特征,其源于地壳变质砂岩部分熔融,形成于同碰撞晚期构造环境,属Rodinia超大陆汇聚阶段的产物。综合研究表明,阿尔金地区新元古代早期同碰撞型岩体的形成时代集中在871～945 Ma,限定了Rodinia超大陆汇聚时限,且在空间上构成了一条重要的岩浆岩带,是对Rodinia超大陆碰撞汇聚作用的响应。     

青海三江北段拉地贡玛地区印支期花岗岩地球化学特征及地质意义

拉地贡玛地区花岗岩处于金沙江缝合带内,区域上隶属西金乌兰—金沙江缝合带的物质组成部分,在研究区属通天河构造岩浆岩带。据出露岩石类型,将其划分为3个单元: 拉地贡玛花岗闪长岩(T₃γδ)、日阿日曲石英闪长岩(T₃δο)、缅切英云闪长岩(T₃δi)。岩浆侵位时代为晚三叠世; 侵入岩体是以高硅富铝、多碱质和挥发组分为特征的兼具I型和S型花岗岩,属过铝质花岗岩; 轻重稀土分馏程度明显,轻稀土富集,Eu、Ce呈弱负异常,Rb、Th、Ba等元素富集,Y、Yb、Cr等元素亏损,具I型和S型花岗岩的特点,与火山弧钙碱性花岗岩特征相似。研究区花岗岩形成构造环境与晚三叠世多彩—当江构造混杂带的俯冲作用有关,是与俯冲汇聚构造环境有关的大陆碰撞弧型花岗岩。     

三江地区中段岩石地球化学分区研究

根据特定的地质构造及幔型元素(KT)特征值(富集系数累加值)的不同，将三江地区中段划分为7个地球化学区，即扬子基底、扬子盖层、金沙江古特提斯、澜沧江古特提斯、兰坪中新生盆地、印支花岗岩、喜山花岗岩地球化学区；再根据各地球化学区中高强场元素(KNb)或放射性元素(KTh)特征值的不同划分为16个子区(或亚区)。进而论述各地球化学子区(或亚区)特征值的变化规律；并寻找成矿元素在某些子区的富集特点。     

内蒙古苇莲河石英脉型黑钨矿赋矿花岗岩成岩时代、地球化学特征及其地质意义

内蒙古苇莲河地区是近年来进行黑钨矿找矿勘查的区域,位于兴蒙造山带中东部,大兴安岭主脊中部。矿床类型可能为典型的石英脉型黑钨矿。黑钨矿石英脉以近于直立（75°）的平直脉体产于花岗岩内或岩体与地层的外接触带,脉体向下尖灭于岩体内部,主要的矿化蚀变类型有钾长石化、绿帘石化、绢云母化、硅化等。本文在初步研究矿床地质特征的基础上,对成矿岩体——花岗岩进行高精度LA-ICP-MS锆石U-Pb测年及地球化学分析。UPb测年结果显示,²⁰⁶Pb/²³⁸U谐和年龄为（145±1）Ma,加权平均年龄为（143±1）Ma。地球化学特征显示,岩体具有典型的富硅、富碱特征,SiO₂平均74.13%,（Na₂O+K₂O）平均9.44%,Na₂O/K₂O平均0.95,属高钾钙碱性系列岩石;Al₂O₃平均为12.91%,铝饱和指数A/CNK全部靠近1.0,为准铝质到弱过铝质花岗岩类,相对贫Mg;稀土总量较低,∑REE变化于122.02×10^-6~235.79×10^-6,表现为右倾稀土元素配分模式,弱富集轻稀土,并具有强烈的负铕异常,高场强元素Th、U、Nd、Hf明显富集,而Ba、Sr、P、Ti、Nb显著亏损;低Sr,高Yb,Nb/Ta比值变化于12.12~13.78,强不相容元素Rb高度富集,Rb/Sr比值1.25~14.58,Ti/Y比值42.49~114.48,Ti/Zr比值5.20~8.45;总体为壳源岩浆的特征。苇莲河花岗岩侵位时代为早白垩世,属燕山期构造岩浆活动的产物,其可能的形成构造背景为古太平洋板块斜向俯冲后,后退过程中的伸展滑塌背景。     

安徽牯牛降A型花岗岩的年代学、地球化学和构造意义

皖南地区牯牛降岩体位于扬子板块东南缘,江南隆起带内。本文报道了牯牛降花岗岩体新的锆石U-Pb年龄和地球化学数据,并对岩体成因及其构造意义进行了探讨。锆石原位LA-ICP-MS U-Pb定年表明牯牛降岩体的形成年龄为130.1±1.3Ma (95% confidence, MSWD=0.55)。结合己发表的其他高质量锆石U-Pb同位素年龄数据表明皖南地区花岗岩的形成年龄主要集中在125~130Ma。牯牛降花岗岩为高钾钙碱性、准铝质岩石,SiO₂ 含量为72.21%~74.85%,具有高K₂O含量(>5.11%)、高铁值(FeO^T/(FeO^T+MgO)>0.91)和K₂O/Na₂O比值(>1.61),低MgO和CaO含量的特征。微量元素地球化学性质上表现为强烈亏损Ba、Sr、Eu(Eu^*/Eu=0.29~0.30),富集REE(>419×10^-6)、Rb、Th 和U,较高的高场强元素Zr、Nb、Y和Ga含量。主量和微量元素均表现为A型花岗岩的特征。非常低的Mg^#值(0.14~0.16)和较低Cr含量(Cr=10×10^-6),高Yb(7.08×10^-6~9.02×10^-6)、Y(78.7×10^-6~90.8×10^-6)含量和较高的Th/U比值(5.17~7.79)说明古老地壳物质的部分熔融可能是牯牛降岩体主要形成机制。牯牛降A2型花岗岩特征代表了拉张的碰撞后构造环境。     

原地-半原地深熔花岗岩特征:以华北克拉通北缘包头地区石榴花岗岩为例

在华北克拉通北缘大青山地区,广泛的深熔作用导致新太古代晚期石榴花岗岩发育。石榴花岗岩空间上与新太古代晚期大青山表壳岩(主要为石榴黑云母片麻岩)共生,渐变过渡。宏观上岩性具有不均一性,在包头哈德门沟一条实测地质剖面上可以观察到石榴混合闪长岩、石榴混合石英闪长岩和石榴混合花岗闪长岩等不同岩石类型。岩相学研究表明,石榴花岗岩与源岩矿物组合相近,但含量却大不相同。石榴花岗岩和源岩中的石榴石特征十分相似。石榴石周围存在放射状黑云母,是典型的逆反应结构。研究表明深熔作用可能发生在具有顺时针P-T轨迹的变质作用演化峰期及峰期后近等温降压阶段,主要发生钾长石熔融和黑云母熔融。地球化学特征研究表明石榴花岗岩与石榴黑云母片麻岩具有亲缘关系,均表现出高Al2O3、CaO含量,K2O/Na2O比值存在较大变化,富集轻稀土和大离子亲石元素,亏损Nb、Ta、P和Ti。但是,石榴花岗岩存在组成变化,Eu富集型和亏损型稀土模式为原地-半原地深熔花岗岩的标志性特征。SHRIMP锆石年代学研究表明深熔作用发生在2.43Ga之前,与区内新太古代晚期水平顺层滑脱变形有关,揭示华北克拉通北缘新太古代晚期构造热事件。