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1.
The gneisses and granitoids of Bastar craton (with rock suites up to 3.5 Ga) show calc-alkaline trondhjemitic characteristics. The rocks are enriched in both LILE and HFSE than primordial mantle. They have also relatively higher abundances of LILE and strong depletion at P and Ti in the multielement diagram. The depletion of Ti and P indicates retention of these elements by titanite and/or apatite during partial melting. It is proposed that subduction of an oceanic slab and its consequent melting led to the formation of the protoliths of the gneisses without much interaction with the mantle wedge. The granitoids represent temporally distinct suites formed in response to further melting of slab at greater depth and interaction of magma with the mantle wedge during their transport to the crust.  相似文献   

2.
《Gondwana Research》2006,9(4):589-595
The Central Indian continental crust is postulated to have formed around the Archean nuclei of the Bastar Craton (Radhakrishna, 1993). Around 3.5 Ga. Old, high-Al 2 O 3 trondhjemite gneisses have been reported from the southern part of the Bastar Craton (Sarkar et al., 1993). However, neither isotopic nor geochemical evidence exists in the literature for the presence of rocks older than ∼2.5 Ga from the northern part of the Bastar Craton (Sarkar et al., 1990). The absence of tonalite-trondhjemite-granodiorite (TTG) suites from the Amgaon Gneisses (Rao et al., 2000), were considered to indicate substantial geochemical differences between the Amgaon gneisses and the TTG basement gneisses of the Dharwar Craton (i.e., the peninsular gneisses). Accordingly the mode of the tectonomagmatic evolutionary patterns of the Bastar Craton was considered to be different, both in time in space from the bordering Dharwar and Bundelkhand Cratons, respectively. In this communication we report the presence of high-Al 2 O 3 trondhjemite from the Amgaon gneisses, along with calc-alkaline and peraluminous granites that are geochemically similar to the late granitoids (∼2.5 to 2.6 Ga old) of the Dharwar Craton, suggesting that the two cratons were nearest neighbours at least during the late Archean.  相似文献   

3.
The Central Indian continental crust is postulated to have formed around the Archean nuclei of the Bastar Craton (Radhakrishna, 1993). Around 3.5 Ga. Old, high-Al 2 O 3 trondhjemite gneisses have been reported from the southern part of the Bastar Craton (Sarkar et al., 1993). However, neither isotopic nor geochemical evidence exists in the literature for the presence of rocks older than 2.5 Ga from the northern part of the Bastar Craton (Sarkar et al., 1990). The absence of tonalite-trondhjemite-granodiorite (TTG) suites from the Amgaon Gneisses (Rao et al., 2000), were considered to indicate substantial geochemical differences between the Amgaon gneisses and the TTG basement gneisses of the Dharwar Craton (i.e., the peninsular gneisses). Accordingly the mode of the tectonomagmatic evolutionary patterns of the Bastar Craton was considered to be different, both in time in space from the bordering Dharwar and Bundelkhand Cratons, respectively. In this communication we report the presence of high-Al 2 O 3 trondhjemite from the Amgaon gneisses, along with calc-alkaline and peraluminous granites that are geochemically similar to the late granitoids (2.5 to 2.6 Ga old) of the Dharwar Craton, suggesting that the two cratons were nearest neighbours at least during the late Archean.  相似文献   

4.
The Dineibit El-Qulieb Leucogranite exhibits most features of l-type granitoids (calc-alkaline affinity, relatively high Na2O, moderate values of Rb, Ba, LREE, Rb/Sr and low Rb/Ba with the presence of magnetite and titanite as the main accessories). On the other hand, they possess hyperaluminous (molar A/CNK = 1.22−1.43) and high normative corundum (∼ 5%), which are in contrast to typical l-type granitoids. The REE patterns are characterised by fractionated LREE and relatively flat HREE with pronounced negative Eu anomalies. The investigated rocks have low K/Rb and high Zr/Y ratios reflecting a typical mature continentalarc environment.The absence of recrystallised phases and the undepleted and flat HREE of the Dineibit El-Qulieb Leucogranite pattern argue against its formation by partial melting of crustal materials. Based on the petrological and geochemical features, the Dineibit El-Qulieb Leucogranite can be generated by fractional crystallisation of mafic magma. The Qulieb leucogranites are characterised by LILE enrichment, normative corundum-rich, strongly peraluminous compositions and associated with miarolitic cavities and pegmatitic patches suggesting the role of the aqueous fluids released from the downgoing slab during subduction. The main fractionating phases were hornblende, biotite, plagioclase and alkali feldspars. Based on the modelling of major elements, the least differentiated adamellite sample requires 91% crystal fractionation, mainly of hornblende, plagioclase, K-feldspar and biotite, from dioritic liquid. On the other hand, the most felsic investigated adamellite sample can be generated by 29% fractional crystallisation of plagioclase, K-feldspar and biotite from the most basic adamellite sample.  相似文献   

5.
The Khawr Fakkan block of the Semail ophiolite (United Arab Emirates) exhibits a suite of 10–100 m scale metaluminous to peraluminous granitic intrusions, ranging from cordierite-andalusite-biotite monzogranites to garnet-tourmaline leucogranites, which intrude mantle sequence harzburgites and lower crustal sequence cumulate gabbros. Structural constraints suggest that the subduction of continental sedimentary material beneath the hot proto-ophiolite in an intra-oceanic arc environment led to granulite facies metamorphism at the subduction front and the generation of granitic melts which were emplaced up to the level of the ophiolite Moho. Compositions indicate the analysed granitoids were largely minimum melts that crystallised at variable a H2O and pressures of 3 to 5 kbar. The LILE (Sr, Rb and Ba) covariation modelling suggests that the granitoids formed largely by the dehydration melting of muscovite rich metasediments. Initial 87Sr/86Sr ratios of analysed dykes vary between 0.710 and 0.706 at initial ɛNd values of between −6.3 and −0.5. Cogenetic units of a composite sill from Ra's Dadnah yield a Sm-Nd isochron age of 98.8 ± 9.5 Ma (MSWD = 1.18). Geochemical and isotopic characteristics of the analysed granitic intrusions indicate that the subducted continental material was derived from oceanic trench fill (Haybi complex) sediments, preserved as greenschist (Asimah area) to granulite facies (Bani Hamid area) ophiolitic metamorphic sole terranes. The Sr-Nd isotope systematics suggest that hybrid granitic melts were derived from pre-magmatic mixing of two contrasting subduction zone sources. Received: 17 December 1998 / Accepted: 19 July 1999  相似文献   

6.
The Baer ophiolitic massif is located in the northern sub-belt of the western segment of the Yarlung Zangbo Suture Zone (YZSZ) and mainly consists of a lherzolite-dominant mantle suite, dolerite intrusions and limited crustal outcrops. The dolerites show sub-ophitic texture and light rare earth element-depleted chondrite-normalized rare earth element patterns similar to normal-mid-ocean ridge basalts (N-MORB); though, they display enrichments in fluid-mobile elements (Rb, Ba, and Sr) and marked depletions in Th and Nb. The U–Pb ages of several magmatic zircon grains recovered from two dolerite samples indicate that the intrusion of the dikes into the Baer lherzolitic mantle occurred at 125.6–126.3 Ma, consistent with the widespread mafic magmatism between 120 and 130 Ma in the Yarlung Zangbo ophiolites. The dolerites have slightly more radiogenic 87Sr/86Sr ratios (0.7043–0.7054) in comparison to N-MORB, whereas they show 143Nd/144Nd values (0.513067–0.513114) similar to N-MORB and high zircon Hf-isotope compositions. They have a limited range of Nd-isotope (εNd(t) values: +8.2 to +9.1) and juvenile Hf-isotope compositions (εHf(t) values: +8.4 to +14.2 and +10.0 to +15.1) indicating derivation from mantle melts. The moderate spread in the εHf (t) values of zircons indicates derivation of the dolerites parental magma from a weakly contaminated spinel-bearing mantle source. This is also corroborated by the geochemical signatures of the Baer dolerites (enrichment in LILE and depletion in HFSE) suggesting minor slab input to the mantle source of the dike-filling melt. We suggest that the genesis of the dolerite dike-forming melt happened at a stage of subduction initiation in a sub-oceanic mantle domain mildly affected by fluids emanating from the downgoing slab. Our data combined with literature data allow us to presume that the intrusion of the dolerites into the Baer mantle corresponds to an early phase of subduction initiation beneath a developing forearc basin.  相似文献   

7.
胡受奚  叶瑛等 《岩石学报》2001,17(3):425-435
海西期末形成的初始欧亚板块，从三叠纪（250Ma)开始，便与古太平洋板块－太平洋板块发生强烈的挤压碰撞作用和俯冲作用，及由此引起的远距离效应使中国广大的东部地区从中生代开始成为活动大陆边缘，俯冲作用及构造环境的演化是控制胶东地体等大多数中生代榴0辉岩等变质岩类，花岗岩类，火山岩类，煌斑岩类以及金等矿床形成和使它们出露地表的重要因素，胶东地体中富钾的钙－碱性煌斑岩类的特征与其它活动大陆边缘和造山带中金矿有关的煌斑岩很相似，例如呈脉状产出，晚期侵位，强烈的自变质，富含大离子亲石元素（LILE）和挥发分，高的金含量，高的LREE／HREE和^87Sr/^86Sr比值，低的^143Nd/^144Nd比值，与花岗岩类，橄榄安粗岩，酸性脉岩以及热液金矿床的密切共生及它们这间与富集地幔楔的成因联系等，这些都表明它们与板块或板片俯冲过程中的去气，去碱，去ILE作用或壳－幔物质交换作用及由此形成的富集地幔楔有着紧密的联系。  相似文献   

8.
Mafic dikes, which transect the Mesoarchaean Singhbhum Granitoid Complex, are the most abundant members of the Newer Dolerite dikes of the Singhbhum Orissa craton. These dikes are subalkaline and exhibit a tholeiitic differentiation trend. Studied dikes underwent fractional crystallization of clinopyroxene and plagioclase. They show enriched patterns for the light rare earth elements (LREE) and large ion lithophile elements (LILE). On primitive mantle-normalized multi-element patterns, they possess Ba, Nb, Sr, P, and Ti depletions similar to subduction-related basaltic rocks. The high (La/Yb) n and (Gd/Yb) n ratios suggest that the studied mafic dikes were derived by low degrees of partial melting of a garnet-bearing source. Judging by trace elemental ratios (e.g. Ba/Y, Nb/Y, Ba/Th and Th/Nb), the studied dikes were derived from a mantle source metasomatized by a subduction component (e.g. fluids derived by dehydration of the subducting slab). We conclude that interaction between these fluids and the overlying mantle was the main cause of (LREE and LILE) enrichment and Nb (high field strength elements) depletion in the mafic dikes.  相似文献   

9.
The Bafoussam area in western Cameroon is part of the Central African Orogenic Belt. It is dominated by granitoids which belong to the Pan-African syn- to post-collisional post-650 Ma group. Syenogranites are predominant, but alkali-feldspar granite, monzogranite, quartz-monzonite and quartz-monzodiorite occur as well. Four granitoid suites, biotite granitoids and deformed biotite granitoids with amphibole, megafeldspar granitoids with megacrysts and two-mica granitoids with primary muscovite and igneous garnet are distinguished. The granites can be assigned to high-K calc-alkalic to shoshonitic series. The partly shoshonitic biotite granitoids are metaluminous to weakly peraluminous and can be labelled as a highly fractionated I-type suite. The megafeldspar granitoids are weakly peraluminous with I-type character whereas the two-mica granitoids are weakly to strongly peraluminous and belong to an S-type suite. Emplacement ages at 558–564 Ma for the two-mica granitoids have been dated from monazite by the EMP Th–U–Pb method.The REE in the biotite granitoids are moderately fractionated with (La/Lu)N = 23–38. Enrichment of Nb and Ta varies by one order of magnitude. The megafeldspar granitoids show homogeneous and strongly fractionated REE patterns with (La/Lu)N = 27–42. The primitive mantle-normalized element patterns are homogeneous with marked negative Ba, Nb, Ta, Sr, Eu and Ti anomalies. The two-mica granitoids are characterized by low to moderate total REE contents with strongly fractionated REE expressed by (La/Lu)N ranging from 7 to 59. The negative Nb and Ta anomalies are less significant. Nd and Sr whole-rock isotope data confirm different sources for the granitoid suites. The source of the I-type biotite granitoids was probably a juvenile mantle which has been variably metasomatized. The source of the I-type megafeldspar granitoids is characterized by juvenile mantle and lower crust components. Anatectic melts of the upper continental crust with variable contribution of lower continental crust or mantle melts can explain the heterogeneous isotopic signatures of the S-type two-mica granitoids. It is suggested that the melting of these sources was successively initiated by the rising isotherms during a syn- to post-collisional setting which followed a subduction.  相似文献   

10.
High-K granitoids are among the most abundant rock types in many Archean cratons. Late Neoarchean monzogranitic to syenogranitic gneisses with high-K affinities are widely distributed in the Anshan, Suizhong, Qinhuangdao, and Aolaishan areas on the northeastern margin of the North China Craton (NCC). In this contribution, we present an integrated study of zircon U–Pb–Hf–O isotopic compositions and whole-rock elemental compositions of amphibolites and trondhjemitic and monzo–syenogranitic gneisses of the Jinzhou area in the metamorphic basement of eastern Hebei–western Liaoning, with the aim of constraining their petrogenesis and geodynamic setting. Emplacement ages of the amphibolites and trondhjemitic–monzogranitic gneisses are 2543 ± 27, 2532 ± 19, and 2513 ± 7 Ma, respectively. The amphibolites are tholeiitic in composition with SiO2 contents of 49.7–50.8 wt%, variable degrees of light rare-earth-element (LREE) enrichment and high-field-strength element (HFSE) depletion, and high zircon εHf(t) values of +2.6 to +6.3, suggesting a depleted lithospheric mantle origin. The major- and trace-element compositions of the trondhjemitic gneisses are similar to those of the low-pressure tonalite–trondhjemite–granodiorite (TTG) suite. The zircon Hf (εHf(t) = +1.6 to +3.9) and O (δ18O = +3.76‰ to +5.73‰) isotopic compositions of the trondhjemitic gneisses indicate a juvenile basaltic source at the base of a thickened magmatic arc. The monzogranitic gneisses differ from their TTG counterparts in that they have lower SiO2 and higher MgO, K2O, and incompatible-element (especially Ba, Th, Sr, P, and LREE) contents. They also have slightly evolved zircon εHf(t) values (+0.6 to +3.8) and higher δ18O values (+4.69‰ to +6.13‰). These features suggest that the monzogranitic gneisses represent sanukitoid-type rocks, with a mantle source modified by crust-derived melts. The weakly deformed syenogranitic gneisses are characterized by high SiO2 and K2O, and very low MgO, Cr, and Ni contents, suggesting that they were formed by partial melting of local TTG rocks. Our results, together with those of previous investigations, suggest that the 2554–2513 Ma low- to high-K magmatism in the Jinzhou area most likely originated in an arc–back-arc tectonic setting on the northern margin of the NCC. The large volumes of high-K granitoids in eastern Hebei–western Liaoning are related to extensive mantle–crust interactions and crustal reworking in such a setting.  相似文献   

11.
In the southern part of the Indian Peninsula, there are a number of alkaline plutons of Proterozoic age. In the northern part of the South Indian granulite terrain, the Yelagiri (syenite, pyroxenite) and Sevattur (syenite, pyroxenite, carbonatite) plutons intrude Archaean epidote-hornblende gneisses. Geochemical and isotopic characteristics of the Yelagiri and Sevattur plutons indicate that the syenitic magmas formed from highly differentiated mantle-derived alkali basalts. The Yelagiri and Sevattur syenites are characterized by evolved Sr and Nd isotopic compositions, pronounced enrichment in LILE and large negative Nb anomalies. Trace element and Sr-Nd isotope characteristics of the Yelagiri and Sevattur syenites are similar to those of the subduction-related alkaline rocks. The scarcity of geological evidence for subduction activity at the time of syenite intrusion during Neoproterozoic does not support a link between the alkaline magmatism and subduction. However, our data are consistent with the model of derivation of the Yelagiri and Sevattur syenites from the subcontinental lithospheric mantle, which was previously enriched by slab derived component. The geochemical and isotopic signatures of other mantle-derived intrusive rocks in the northern part of the South Indian granulite terrain with ages ranging from ca. 2.5 Ga to 0.75 Ga also support the above idea.  相似文献   

12.
This study presents new geochemical data on rocks from the Vespor suite, an important mafic unit from the Juruena arc, Roosevelt-Juruena terrain, SW Amazonian craton, northwest Mato Grosso, Brazil, attempting to define their tectonic setting and type of mantle source. The Juruena arc may be part of a magmatic belt (Jamari and Juruena arcs) at the southwestern Amazonian craton during assembly of the Columbia supercontinent. The investigated rocks represent a Paleoproterozoic subduction-related mafic suite of sigmoidal bodies, composed mainly of gabbro, norite, gabbronorite and diorite, that underwent amphibolite facies metamorphism. Here we present also preliminary petrology aspects and zircon U–Pb geochronology. Geochemical character and variation trends of major and trace elements as well as selected trace element ratios suggest that Vespor suite rocks have a tholeiitic lineage of arc affinity controlled by fractional crystallization with a prominent iron enrichment trend. Gabbros, norites and gabbronorites are characterized by enrichment of LILE and weakly to moderately differentiated HFSE patterns, suggesting their deviation from an enriched heterogeneous lithospheric mantle source. Vespor suite rocks are characterized by depletion of Nb–Ta, P and Ti, with flat distribution of HFSE, markedly large variations in most of the LILE, positive anomalies displayed by Ba, K, Th, Sr, Pb and weak negative anomalies of Hf–Zr. These features reflect limited degrees of crustal contamination associated with a subduction-related magma process where the mantle wedge was chemically modified. In addition, the enrichment in LILE and Pb, low values of the ratios (Lan/Smn – 0.83 to 4.58) and (Nbn/Lan – 0.04 to 0.45), but high Th/Yb ratios, gently to moderately sloping REE profiles (La/Ybn = 2.53–7.37), negative anomalies in HFSE (Ta, Zr, Hf, and Ti), and positive anomalies in LILE (Th, Ba, Sr), suggest derivation from a metasomatized lithospheric mantle source above a subduction zone with weak crustal contamination. Both the composition of the mantle source and the degree of partial melting that produced the parental magmas of these rocks, determined by using REE abundance and ratios, indicate that gabbroic/dioritic melts were generated at different degrees of melting of the source: about 5–20% partial melting of a garnet-spinel lherzolite, around 1–10% partial melting of spinel lherzolite source, and approximately 1–5% partial melting of intermediate source composition, and crystallizing between 1.773 and 1.764 Ma.  相似文献   

13.
Mildly deformed granitoids exposed around Bilgi in the northernmost part of the eastern Dharwar craton are divided into two groups viz. granodiorites and monzogranites. The granodiorites contain microgranular enclaves and amphibolite xenoliths, and show low-Al TTG affinity with high SiO2 (71–74 %), Na2O, Y and Sr/Y, moderate to moderately high Mg#, Cr and Ni, low to moderate LILE, and low Nb and Ta. However, compared to similar TTGs from different cratons the Bilgi granodiorites have distinctly higher K2O, K2O/Na2O, Rb and lower REE and Th. The amphibolite xenoliths are characterized by variable enrichment of K2O, Rb, Ba and Th and depletion of Ti, Zr and P compared to MORB. The microgranular enclaves are quartz diorite to granodiorite in composition with high Mg, Ni and Cr, and compared to MORB, are enriched in LILE and depleted in Ti and Y. The monzogranites, compared to the granodiorites, display higher SiO2, K2O and Rb with lower Mg#, although still maintaining the high Na2O, Ni and Cr and low REE character. The Bilgi granodiorites are explained as transitional TTGs late synkinematic with respect to regional deformation. Geochemical signatures and regional geological set up suggest that they are probably derived from partial melting of a highly depleted slab material (metabasalt) followed by variable contamination or assimilation of intermediate crustal rocks in a subduction zone set up. Late stage fluid activity on the granodioritic magma is probably responsible for the generation of monzogranites. The amphibolite xenoliths predate the granodiorites and possibly represent fragments of a schist belt carried away by the granitic magma. They are probably island arc basalt derived from mantle source that has been metasomatized by slab-derived fluids. The microgranular enclaves are coeval with the Bilgi granodiorites and also likely to be island arc magmas derived from mantle variably enriched in slab-derived and within-plate components.  相似文献   

14.
The Neoarchaean Tati granite–greenstone terrane occurs within the southwestern part of the Zimbabwe craton in NE Botswana. It comprises 10 intrusive bodies forming part of three distinct plutonic suites: (1) an earlier TTG suite dominated by tonalites, trondhjemites, Na-granites distributed into high-Al (Group 1) and low-Al (Group 2) TTG sub-suite rocks; (2) a Sanukitoid suite including gabbros and Mg-diorites; and (3) a younger high-K granite suite displaying I-type, calc-alkaline affinities.

The Group 1 TTG sub-suite rocks are marked by high Sr/Y values and strongly fractionated chondrite-normalized rare earth element (REE) patterns, with no Eu anomaly. The Group 2 TTG sub-suite displays higher LREE contents, negative Eu anomaly and small to no fractionation of HREE. The primordial mantle-normalized patterns of the Francistown TTGs are marked by negative Nb–Ti anomalies. The geochemical characteristics of the TTG rocks are consistent with features of silicate melts from partial melting of flat subducting slabs for the Group 1 sub-suite and partial melting of arc mafic magmas underplated in the lower crust for the Group 2 sub-suite. The gabbros and high-Mg diorites of the Sanukitoid suite are marked by Mg#>0.5, high Al2O3 (>>16%), low TiO2 (<0.6%) and variable enrichment of HFSE and LILE. Their chondrite-normalized REE patterns are flat in gabbros and mildly to substantially fractionated in high-Mg diorites, with minor negative or positive Eu anomalies. The primordial mantle-normalized diagrams display negative Nb–Ti (and Zr in gabbros) anomalies. Variable but high Sr/Y, Sr/Ce, La/Nb, Th/Ta and Cs/La and low Ce/Pb ratios mark the Sanukitoid suite rocks. These geochemical features are consistent with melting of a sub-arc heterogeneously metasomatised mantle wedge source predominantly enriched by earlier TTG melts and fluids from dehydration of a subducting slab. Melting of the mantle wedge is consistent with a steeper subduction system. The late to post-kinematic high-K granite suite includes I-type calc-alkaline rocks generated through crustal partial melting of earlier TTG material. The Neoarchaean tectonic evolution of the Zimbabwe craton is shown to mark a broad continental magmatic arc (and related accretionary thrusts and sedimentary basins) linked to a subduction zone, which operated within the Limpopo–Shashe belt at 2.8–2.65 Ga. The detachment of the subducting slab led to the uprise of a hotter mantle section as the source of heat inducing crustal partial melting of juvenile TTG material to produce the high-K granite suite.  相似文献   


15.
微量元素模拟限定大别造山带中生代花岗岩类的源岩成分   总被引：1，自引：1，他引：0  
徐启东 《现代地质》1997,11(1):48-57
摘 要　 大陆碰撞造山带中花岗岩浆主要是因下部地壳缺乏流体熔融过程而形成的。对其源 岩成分限制而进行的微量元素模拟不仅要考虑源岩和熔体中有关矿物相比例的变化‚还要考 虑熔融过程中源岩矿物相组合的变化‚并选择合适的元素用于模拟。Rb、Sr、Ba 和 REE 模拟 结果显示‚上溪群杂砂岩作为扬子隆起带中生代花岗岩类的源岩是不合适的：大别隆起带中 的中生代花岗岩类也不可能完全由大别杂岩中的 TTG 质片麻岩熔融形成‚更可能是变基性岩 和 TTG 质片麻岩构成的复合源岩熔融的结果。  相似文献   

16.
Melting of subducting oceanic lithosphere and associated melt-mantle interactions in convergent plate margins require specific geodynamic environment that allows the oceanic slab to be abnormally heated. Here we focus on the Early Mesozoic mafic rocks and granite porphyry, which provide insights into slab melting processes associated with final closure of the Paleo-Asian Ocean. The granite porphyry samples are calc-alkaline and distinguished by high Sr contents, strong depletion of heavy rare earth elements, resulting in high (La/Yb)N and Sr/Y ratios, and negligible Eu anomalies. Based on their high Na2O and MgO, low K2O contents, positive εHf(t) and εNd(t) and low (87Sr/86Sr)i values, we propose that the granite porphyry was likely derived from partial melting of subducting Paleo-Asian oceanic crust. The Nb-enriched mafic rocks are enriched in Rb, Th, U, Pb and K, and depleted in Nb, Ta, Ba, P and Ti, corroborating a subduction-related origin. Their heterogeneous Sr-Nd-Hf-O isotopic compositions and other geochemical features suggest that they were likely derived from partial melting of peridotitic mantle wedge interacted with oceanic slab-derived adakitic melts. Trace element and isotope modeling results and elevated zircon δ18O values suggest variable subducting sediments input into the mantle wedge, dominated by terrigenous sediments. Synthesizing the widely-developed bimodal rock associations, conjugated dikes, thermal metamorphism, tectonic characteristics, paleomagnetic constraints, and paleogeographical evidence along the Solonke-Changchun suture zone, we identify a slab window triggered by slab break-off, which accounts for slab melting and formation of the Nb-enriched mafic rocks and associated adakitic granite porphyry in southeastern Central Asian Orogenic Belt.  相似文献   

17.
鄂尔多斯西南缘地处北秦岭、北祁连、贺兰山构造带交接部位,构造带西段的南华山弧形构造带中发育有大量早古生代花岗岩体（岩株）,侵入于下元古界海原群以及中寒武统香山群、下奥陶统阴沟群中。岩性以花岗闪长岩为主,其次为二长花岗岩和石英闪长岩,并有少量角闪奥长花岗岩和斑状花岗岩等,其中普遍含有暗色闪长质包体。地球化学研究表明,岩石主要为准铝-过铝质、钙碱性系列,具有明显高Al2O3、低MgO特征,尤其是低Y和Yb、高Sr和Sr/Y比值特征,类似于典型的埃达克岩地球化学组成。同时,岩石明显亏损Nb、Ta、P 和Ti,富集Cs、Rb、Ba、Th、U、K、Pb、Nd、Zr 等。综合分析认为,早古生代祁连俯冲洋壳板片熔融,熔体与楔形地幔熔体混合后上侵,并与下地壳熔体再次混合形成了南华山花岗岩。利用LA-ICPMS法测得花岗岩锆石U-Pb同位素年龄为416±14 Ma,代表了花岗岩结晶年龄。综合利用区域地质、地层特征和不整合接触关系,限定祁连-秦岭交接区俯冲、碰撞造山和地壳加厚发生在志留纪末期。  相似文献   

18.
Voluminous granitoids are widely distributed in the Langshan region, northeast of the Alxa block, and record the evolutionary processes of the southern Central Asian Orogenic Belt. The Dabashan pluton was emplaced into the Paleoproterozoic Diebusige complex. Early Carboniferous zircon LA-ICP MS U-Pb ages were from 327 Ma to 346 Ma. The Dabashan pluton can be classified as monzogranite and syenogranite, and exhibits high K2O contents and K2O/Na2O ratios, which reveal a high-K calc-alkaline nature. The samples display strongly fractionated REE patterns, and are enriched in large ion lithophile elements (LILE) relative to high field strength elements (HFSE). The Dabashan plutons display unusually high Ba (823–2817 ppm) and Sr (166–520 ppm) contents and K/Rb ratios (315–627), but low Rb/Ba ratios (0.02–0.14), and exhibit fertile zircon Hf isotopic compositions [εHf(t)=?14 to ?20], which are comparable to those of typical high Ba–Sr granitoids. Based on the geochemical compositions of the samples, we suggest that subducted sediments and ancient crustal materials both played important roles in their generation. Basaltic melts were derived from partial melting of subcontinental lithophile mantle metasomatized by subducted sediment-related melts with residual garnet in the source, which caused partial melting of ancient lower crust. Magmas derived from underplating ascended and emplaced in the middle–upper crust at different depths. The resultant magmas experienced some degree of fractional crystallization during their ascent. Given these geochemical characteristics, together with regional tectonic, magmatic, and structure analysis data, an active continental margin environment is proposed for the generation of these rocks.  相似文献   

19.
Tectonic processes involving amalgamations of microblocks along zones of ocean closure represented by granite-greenstone belts(GGB) were fundamental in building the Earth's early continents. The crustal growth and cratonization of the North China Craton(NCC) are correlated to the amalgamation of microblocks welded by 2.75-2.6 Ga and ~2.5 Ga GGBs. The lithological assemblages in the GGBs are broadly represented by volcano-sedimentary sequences, subduction-collision related granitoids and bimodal volcanic rocks(basalt and dacite) interlayered with minor komatiites and calc-alkalic volcanic rocks(basalt, andesite and felsic rock). The geochemical features of meta-basalts in the major GGBs of the NCC display affinity with N-MORB, E-MORB, OIB and calc-alkaline basalt, suggesting that the microblocks were separated by oceanic realm. The granitoid rocks display arc signature with enrichment of LILE(K,Rb, Sr, Ba) and LREE, and depletion of HFSE(Nb, Ta, Th, U, Ti) and HREE, and fall in the VAG field. The major mineralization includes Neoarchean BIF-type iron and VMS-type Cu-Zb deposits and these,together with the associated supracrustal rocks possibly formed in back-arc basins or arc-related oceanic slab subduction setting with or without input from mantle plumes. The 2.75-2.60 Ga TTG rocks,komatiites, meta-basalts and metasedimentary rocks in the Yanlingguan GGB are correlated to the upwelling mantle plume with eruption close to the continental margin within an ocean basin. The volcanosedimentary rocks and granitoid rocks in the late Neoarchean GGBs display formation ages of 2.60-2.48 Ga, followed by metamorphism at 2.52-2.47 Ga, corresponding to a typical modern-style subduction-collision system operating at the dawn of Proterozoic. The late Neoarchean komatiite(Dongwufenzi GGB), sanukitoid(Dongwufenzi GGB and Western Shandong GGB), BIF(Zunhua GGB) and VMS deposit(Hongtoushan-Qingyuan-Helong GGB) have closer connection to a combined process of oceanic slab subduction and mantle plume. The Neoarchean cratonization of the NCC appears to have involved two stages of tectonic process along the 2.75-2.6 Ga GGB and ~2.5 Ga GGBs, the former involve plume-arc interaction process, and the latter involving oceanic lithospheric subduction, with or without arcplume interaction.  相似文献   

20.
The results of field, petrographic and geochemical work of the granitoids of Hutti-Gurgunta area in the northern part of Eastern Dharwar Craton (EDC) is presented in this paper. This crustal section comprises polyphase banded to foliated TTG gneisses, middle amphibolite facies Gurgunta schist belt and upper greenschist facies Hutti schist belt and abundant granite plutons. The focus of the present study is mainly on basement TTG gneisses and a granite pluton (∼ 240 sq km areal extent), to discuss crustal accretion processes including changing petrogenetic mechanism and geodynamic setting. The TTGs contain quartz, plagioclase, lesser K-feldspar and hornblende with minor biotite while the granite contain quartz, plagioclase, K-feldspar and hornblende. Late stage alteration (chloritisation, sericitisation and epidotisation) is wide spread in the entire area. A huge synplutonic mafic body which is dioritic to meladioritic in composition injects the granite and displays all stages of progressive mixing and hybridization. The studied TTGs and granite show distinct major and trace element patterns. The TTGs are characterized by higher SiO2, high Al2O3, and Na2O, low TiO2, Mg#, CaO, K2O and LILE, and HFS elements compared to granite. TTGs define strong trondhjemite trend whilst granite shows calc-alkaline trend. However, both TTGs and granite show characteristics of Phanerozoic high-silica adakites. The granite also shows characteristics of transitional TTGs in its high LILE, and progressive increase in K2O with differentiation. Both TTGs and granite define linear to sub-linear trends on variation diagrams. The TTGs show moderate total REE contents with fractionated REE patterns (La/YbN =17.73–61.73) and slight positive or without any significant Eu anomaly implying little amount of amphibole or plagioclase in residual liquid. On the other hand, the granite displays poor to moderate fractionation of REE patterns (La/YbN = 9.06–67.21) without any significant Eu anomaly. The TTGs have been interpreted to be produced by low-K basaltic slab melting at shallow depth, whereas the granite pluton has been formed by slab melting at depth and these melts interacted with peridotite mantle wedge. Such changing petrogenetic mechanisms and geodynamic conditions explain increase in the contents of MgO, CaO, Ni and Cr from 2700 Ma to 2500 Ma granitoids in the EDC.  相似文献   

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