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1.
The Egyptian older and younger granitic rocks emplaced during pre- and post-collision stages of Neoproterozoic Pan-African orogeny, respectively, are widely distributed in the southern Sinai Peninsula, constituting 70% of the basement outcrops. The Wadi El-Akhder, southwestern Sinai, is a mountainous terrain exposing two granitoid suites, namely the Wadi El-Akhder Older Granites (AOG) and the Homra Younger Granites (HYG). The AOG (granodiorites with subordinate tonalite compositions) have geochemical characteristics of medium-K calc-alkaline, metaluminous to mildly peraluminous granitoids formed in an island-arc environment, which are conformable with well-known Egyptian older granitoids rocks, whereas the HYG display calc-alkaline to slightly alkaline nature, peraluminous syeno-, monzogranites and alkali feldspar granites matching well those of the Egyptian younger granites. With respect to the AOG granitoids, the HYG granites contain lower Al2O3, FeO*, MgO, MnO, CaO, TiO2, Sr, Ba, and V, but higher Na2O, K2O, Nb, Zr, Th, and Rb. The AOG are generally characterized by enrichment in LILE and LREE and depletion in HFSE relative to N-MORB values (e.g., negative Nb and Ta anomalies). The geochemical features of the AOG follow assimilation-fractional crystallization (AFC) trends indicative of extensive crustal contamination of magma derived from a mantle source. The chemical characteristics of the AOG are remarkably similar to those of subduction-related granitoids from the Arabian-Nubian Shield (ANS). The compositional variations from monzogranites through syenogranites to alkali feldspar granite within HYG could not be explained by fractional crystallization solely. Correlating the whole-rock composition of the HYG to melts generated by experimental dehydration melting of meta-sedimentary and magmatic rocks reveals that they appear to be derived by extended melting of psammitic and pelitic metasediments, which is similar to the most of younger granitic suites in the ANS.  相似文献   

2.
本文对藏南冈底斯带中段的花岗岩类和角闪辉长岩进行了锆石U-Pb年代学和全岩地球化学分析,据此阐明了岩体的形成机制与演化过程,并探讨了成岩时的大地构造背景。分析结果显示,研究区内花岗岩类和角闪辉长岩体的LA-ICPMS锆石U-Pb定年结果为41~55Ma,为始新世早-中期岩浆活动的产物,代表了区内岩体的成岩年龄。在地球化学组成上,花岗岩类属于钙碱性到高钾钙碱性系列,均富集轻稀土(LREE)和大离子亲石元素(LILE)(Rb、Ba和K),强烈亏损Nb、Ta、P等高场强元素(HFSE),具有弧型岩浆岩的地球化学组成。此外,花岗岩类的铝饱和指数(A/CNK)小于1.1,属于准铝质到弱过铝质的I型花岗岩。角闪辉长岩为石榴橄榄岩部分熔融的产物,并在后期侵位的过程中遭受到了壳源物质的混染。综合分析表明,研究区内的岩体形成于初始碰撞向主碰撞的转化阶段。始新世早期(～50Ma)新特提斯洋板片的断离引起软流圈物质上涌,导致岩石圈地幔发生部分熔融形成基性岩浆,随后基性岩浆底侵至下地壳并诱发下地壳发生部分熔融形成花岗岩质岩浆,最后经过岩浆混合作用形成始新世早-中期冈底斯地区的花岗岩类。  相似文献   

3.
黑龙江省嫩江至黑河一带发育大量的晚古生代花岗岩类岩石,同位素测年结果显示主要集中在3个时代:早石炭世、晚石炭世和早二叠世。岩石类型从花岗闪长岩-二长花岗岩-正长花岗岩-碱长花岗岩等均有不同出露,石炭纪部分岩石遭受韧性剪切变质变形作用改造形成花岗质糜棱岩。岩石整体具有高硅、富钾钠特征;稀土元素总量偏高,铕负异常明显;微量元素具明显的大离子亲石元素(LILE)K、Rb、Th富集和高场强元素(HFSE)Nb、P、Ti亏损特征。其中石炭纪花岗岩类地球化学特征显示陆缘弧及同碰撞花岗岩特点,二叠纪花岗岩则表现后造山花岗岩特点。二者反映了由碰撞造山向板内后造山阶段转变的构造环境特点,也反映了嫩江-黑河构造带汇聚拼合与伸展的活动史。  相似文献   

4.
滇西保山地块早古生代花岗岩类的年代学、地球化学及意义   总被引：24，自引：12，他引：12  
位于滇西特提斯构造带保山地块的平河岩体,岩石类型主要为花岗岩、二长花岗岩,其次有少量的花岗闪长岩。4件样品的锆石U-Pb年龄变化于480～486Ma,表明这些花岗岩类侵位于早奥陶世。平河花岗岩类的K2O/Na2O值大于1,铝饱和指数(A/CNK)为1.07～1.1,属高钾钙碱性过铝质花岗岩。岩石总体上富集大离子亲石元素和Pb,亏损高场强元素。明显富集轻稀土元素[(La/Yb)N=4.33～7.05],显示明显的负Eu异常(δEu=0.25～0.48)。4件样品58个测点的锆石εHf(t)值变化范围较大(主要集中于-12.4～-3.0之间),对应的Hf同位素地壳模式年龄集中于2.2～1.7Ga。这些地球化学特征指示平河花岗岩类为S型花岗岩,主要来源于古老地壳物质(如砂屑岩)的重熔,并不同程度地混入了幔源物质。平河花岗岩类与出露于中部拉萨地体的变质酸性火山岩存在可比性,可能代表了早古生代冈瓦纳大陆原特提斯边缘岩浆弧的一部分。  相似文献   

5.
In the central segment of the Ribeira belt, southeast Brazil, several foliated porphyritic granitic bodies intrude high-grade migmatitic gneisses of the Andrelândia and Juiz de Fora domains and Embú Complex. Results of geological, geochemical and geochronological investigations of the Maromba, Pedra Selada, Serra do Lagarto and Funil porphyritic I-type granites provide profound similarities, except for the distinct geochemical behavior of the Funil Granite, perhaps related to a different crustal source. These granitoids show similar structural, textural and mineralogical features. Pb-evaporation of single zircons provided ages of 586 ± 6, 579.6 ± 6.3, 586.3 ± 4.8 and 584 ± 5 Ma for the granites, respectively, coincident with the syn-collision I episode of the central Ribeira belt. The intrusion of I-type porphyritic granitoids coeval with the main collisional event has not often been reported in the geological literature. The most common syn-collisional granitic magmatism has normally an S-type signature or even a slightly peraluminous I-type character. However, the occurrence of coeval I- and S-type syn-tectonic granites along the central Ribeira belt, as observed in the investigated area and discussed in this paper is noteworthy.  相似文献   

6.
The evolution of Late Paleozoic granitoid magmatism in Transbaikalia shows a general tendency for an increase in the alkalinity of successively forming intrusive complexes: from high-K calc-alkaline granites of the Barguzin complex (Angara–Vitim batholith) at the early stage through transitional from calc-alkaline to alkaline granites and quartz syenites (Zaza complex) at the intermediate stage to peralkaline granitoids (Early Kunalei complex) at the last stage. This evolution trend is complicated by the synchronous development of granitoid complexes with different sets and geochemical compositions of rocks. The compositional changes were accompanied by the decrease in the scales of granitoid magmatism occurrence with time. Crustal metaterrigenous protoliths, possibly of different compositions and ages, were the source of granitoids of the Angara–Vitim batholith. The isotopic composition of all following granitoid complexes points to their mixed mantle–crustal genesis. The mechanisms of granitoid formation are different. Some granitoids formed through the mixing of mantle and crustal magmas; others resulted from the fractional crystallization of hybrid melts; and the rest originated from the fractional crystallization of mantle products or the melting of metabasic sources with the varying but subordinate contribution of crustal protoliths. Synplutonic basic intrusions, combined dikes, and mafic inclusions, specific for the post-Barguzin granitoids, are direct geologic evidence for the synchronous occurrence of crustal and mantle magmatism. The geodynamic setting of the Late Paleozoic magmatism in the Baikal folded area is still debatable. Three possible models are proposed: (1) mantle plume impact, (2) active continental margin, and (3) postcollisional rifting. The latter model agrees with the absence of mafic rocks from the Angara–Vitim batholith structure and with the post-Barguzin age of peralkaline rocks of the Vitim province.  相似文献   

7.
Granitoid intrusions occur widely in the Southern Granulite Terrain (SGT) of India, particularly within the Cauvery Suture Zone (CSZ), which is considered as the trace of the Neoproterozoic Mozambique ocean closure. Here we present the petrological and geochemical features of 19 granite plutons across the three major tectonic blocks of the terrain. Our data show a wide variation in the compositions of these intrusions from alkali feldspathic syenite to granite. The whole rock geochemistry of these intrusions displays higher concentrations of \(\hbox {SiO}_{2}\), FeO*, \(\hbox {K}_{2}\hbox {O}\), Ba, Zr, Th, LREE and low MgO, \(\hbox {Na}_{2}\hbox {O}\), Ti, P, Nb, Y and HREE’s. The granitoids are metaluminous to slightly peraluminous in nature revealing both I-type and A-type origin. In tectonic discrimination plots, the plutons dominantly show volcanic arc and syn-collisional as well as post-collisional affinity. Based on the available age data together with geochemical constrains, we demonstrate that the granitic magmatism in the centre and south of the terrain is mostly associated with the Neoproterozoic subduction–collision–accretion–orogeny, followed by extensional mechanism of Gondwana tectonics events. Similar widespread granitic activity has also been documented in the Arabian Nubian shield, Madagascar, Sri Lanka and Antarctica, providing similarities for the reconstruction of the crustal fragments of Gondwana supercontinent followed by Pan-African orogeny.  相似文献   

8.
The northern part of the central India tectonic zone (CITZ) is occupied by the Proterozoic Mahakoshal Belt, which is mainly comprised of granitoids and volcano-sedimentary lithounits. The granitoids (ca. 1880–1710 Ma) are exposed as small circular to elliptical-shaped, stock-like intrusive bodies, such as Nerueadamar granitoids (NG), Tumiya granitoids (TG), Jhirgadandi granitoids (JG), Dudhi granite gneiss (DG), Raspahari granitoids (RG), Katoli granitoids (KG), and Harnakachar granitoids (HG), collectively forming the granite gneissic complex (GGC). The geochemistry of biotites, host granitoids, and enclaves from these plutons has been investigated in order to understand the redox condition and likely tectonic affinity of host granitoids. The Al2O3–MgO–FeOt contents and operated elemental substitution in biotites strongly suggest the diverse nature of host magmas such as calc-alkaline, metaluminous (I-type), peraluminous (S-type), and transitional between I- and S-types, which appear to have formed in subduction zone and syn-collisional tectonic settings. The transitional (I-S)-type granitoids inferred based on biotite compositions, however, represent both metaluminous (HG) and peraluminous (DG and KG) granitoids in terms of whole-rock molar A/CNK (Al2O3/CaO + Na2O + K2O) ratios. Ages of granitoid magmatism and its field association with contemporaneous volcano-sedimentary lithounits clearly mark the post-collisional tectonic setting, which contradicts the subduction-related tectonic setting inferred from biotites of JG and microgranular enclave (JE) hosted in JG. Whole-rock major and trace elements broadly suggest the existence of collision tectonics during the formation of granitoid plutons. The JG, KG, and DG contain a bt-Kf-mag-qtz assemblage, and their parental magmas evolved under moderate oxidizing environments (?O2 = ?12.03 to ?13.27 bars). On the other hand, RG (bt-gt-Kf-pl-qtz), NG (bt-ms-Kf-pl-qtz), and TG (bt-ms-Kf-pl-qtz) represent pure crustal-derived magmas evolved in strongly reducing conditions formed under a syn-collisional tectonic setting as evident from their mineral assemblages and biotite and whole-rock compositions. Granitoid plutons of the Mahakoshal Belt were most likely formed during amalgamation of the Columbian supercontinent.  相似文献   

9.
Garnet, an uncommon accessory mineral in igneous rocks, occurs in seven small peraluminous granitoid plutons in the southeastern Arabian Shield; textural equilibrium between garnet and other host granitoid minerals indicates that the garnets crystallized from their host magmas. Compositions of the garnets form three groups that reflect host-granitoid compositions, which in turn reflect source compositions and tectonic regimes in which the host magmas were generated. Garnets from the seven plutons have almandine-rich cores and spessartine-rich rims. This reverse zoning depicts host magma compositional evolution; i.e. rimward spessartine enrichment resulted from progressive, host-magma manganese enrichment. The garnets are heavy rare-earth element enriched; (Lu/La) N ranges from 13 to 355 and one of the garnets contains spectacularly elevated abundances of Y, Ta, Th, U, Zn, Zr, Hf, Sn, and Nb. Involvement of garnets with these trace element characteristics in magma genesis or evolution can have dramatic effects on trace element signatures of the resulting magmas. Other researchers suggest that Mn-enriched magmas are most conducive to garnet nucleation. Although the garnetiferous granitoids discussed here are slightly Mn enriched, other genetically similar peraluminous Arabian granitoids lack garnet; Mn enrichment alone does not guarantee garnet nucleation. The presence of excess alumina in the magma may be a prerequisite for garnet nucleation.  相似文献   

10.
1INTRODUCTION THELATEPRECAMBRIANGRANITOIDSOFTHEARABONU BIANSHIELDINEGYPTWEREEXPOSEDBYEARLYTOMIDDLE TERTIARYUPLIFTANDENSUINGEROSIONDURINGTHEREDSEA RIFTINGEVENT(GREENBERG,1981).THEREAREANUMBER OFEFFECTIVEANDRELATIVELYSUCCESSFULSCHEMESFORTHE CLASSIFICATIONOF…  相似文献   

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