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1.
Devonian basaltic to andesitic dykes and compositionally similar plutons of the southern Lachlan Fold Belt are often temporally and spatially closely associated with large granitic complexes. Mafic intrusions play a major role in the transfer of heat into the continental crust, providing a thermal ‘engine’ which leads to crustal melting, and geochemical/isotopic evidence indicates that they contribute chemical constituents to the products of this melting. Studied mafic‐intermediate dykes in the southern Lachlan Fold Belt have tholeiitic to alkaline affinities and include groups with both high and low Ti and K. Several dyke generations may be associated with a single felsic complex. Primitive mantle‐normalised trace‐element abundance patterns with negative Nb and Ti anomalies for basaltic/andesitic and gabbroic/dioritic rocks as young as Early Devonian most resemble those of modern island arcs and suggest an influence of subduction on mantle magma sources. However, some Middle and Late Devonian mafic rocks are enriched in light rare‐earth elements and other incompatible elements, lack significant Nb anomalies, and confirm the change to continental‐rift extensional settings clearly indicated by Lachlan Fold Belt geology.  相似文献   

2.
The mafic dyke swarms are important feature of the Proterozoic and in parts of some stabilised cratonic areas. The early Proterozoic Bundelkhand massif of Central India is extensively intruded by suites of NW-SE and NE-SW trending mafic and ultramafic dykes. These dykes are mostly dolerites with subordinate pyroxenite, or lamproites, moreover, geochemical signatures of the two compositional types are different for the NW-SE and NE-SW trending suites. 40Ar/39Ar age determinations of the dolerite dykes suggest two phases of dyke activity at c.2150Ma and c.2000 Ma in this region. The dolerites are typically tholeiites and quartz normative types represented by Group I and Group II, whilst the ultramafics are komatiite or basaltic komatiite in composition and show an olivine-normative character. Rare earth element (REE) patterns show some enrichment of LREE and exhibit both positive and negative Eu anomalies. Most of the tholeiites display incompatible elements patterns indicative of an enriched mantle source, whilst those of the ultramafics indicate a depleted source. The 2 Ga event is a global event and well documented in various parts of Singhbhum, Aravalli terrane, Tamilnadu, Andhra Pradesh and Kerala regions of Indian Peninsular Shield and many parts of globe. The genesis of these dyke swarms clearly constitutes a major thermal event affecting the Earth's mantle during that period.  相似文献   

3.
Equilibrium melt trace element contents are calculated from Proterozoic Nain Plutonic Suite (NPS) mafic and anorthositic cumulates, and from plagioclase and orthopyroxene megacrysts. Assumed trapped melt fractions (TMF) <20% generally eliminate all minor phases in most mafic cumulate rocks, reducing them to mixtures of feldspar, pyroxene and olivine, which would represent the high-temperature cumulus assemblage. In anorthosites, TMF <15% generally reduce the mode to a feldspar-only assemblage. All model melts have trace element profiles enriched in highly incompatible elements relative to normal mid-ocean ridge basalt (NMORB); commonly with negative Nb and Th anomalies. Most mafic cumulates yield similar profiles with constant incompatible element ratios, and can be linked through fractional crystallization. High K-La subtypes probably represent crust-contaminated facies. Mafic cumulates are inferred to belong to a tholeiitic differentiation series, variably contaminated by upper and lower crustal components, and probably related to coeval tholeiitic basaltic dyke swarms and lavas in Labrador. Model melts from anorthosites and megacrysts have normalized trace element profiles with steeper slopes than those calculated from mafic cumulates, indicating that mafic cumulates and anorthosites did not crystallize from the same melts. Orthopyroxene megacrysts yield model melts that are more enriched than typical anorthositic model melts, precluding an origin from parental melts. Jotunites have lower K-Rb-Ba-Y-Yb and higher La-Ce than model residues from fractionation of anorthositic model melts, suggesting they are not cosanguineous with them, but provide reasonable fits to evolved mafic cumulate model melts. Incompatible element profiles of anorthositic model melts closely resemble those of crustal melts such as tonalites, with steep Y-Yb-Lu segments that suggest residual garnet in the source. Inversion models yield protoliths similar to depleted lower crustal granulite xenoliths with aluminous compositions, suggesting that the incompatible trace element budget of the anorthosites are derived from remobilization of the lower crust. The similarity of the highly incompatible trace elements and LILE between anorthositic and mafic cumulate model melts suggests that the basalts parental to the mafic cumulates locally assimilated considerable quantities of the same crust that yielded the anorthosites. The reaction between underplating basalt and aluminous lower crust would have forced crystallization of abundant plagioclase, and remobilization of these hybrid plagioclase-rich mushes then produced the anorthosite massifs.  相似文献   

4.
The mafic dykes in Wadi Mandar-Wadi Um Adawi area are as follows: (1) calc-alkaline lamprophyre (i.e., kersantite and spessartite), (2) diabase, and (3) alkaline lamprophyre (i.e., camptonite). The field relations reveal that the emplacement of calc-alkaline lamprophyres preceded the diabase dykes, while alkaline lamprophyres emplaced later than the diabase dykes. Calc-alkaline are basaltic andesite, basaltic trachyandesite to basalt, while the diabase dykes and alkaline lamprophyres are basaltic in composition. These dykes are characterized by metaluminous character. Calc-alkaline lamprophyres and diabase dykes show transitional affinity from calc-alkaline to alkaline, while the alkaline lamprophyres exhibit more strong alkaline character. The mafic dykes were crystallized under temperature 1100–1150 °C and pressure 3–5 kbars in a high oxygen fugacity conditions. Fe-Ti oxides in the dykes are represented by ilmenite and Ti-magnetite. The chemistry of the sulfides hosted in those mafic dykes suggests a magmatic-hydrothermal origin for these minerals. The geochemical behavior of high field strength elements and large ion lithophile elements in these dykes excludes the derivation of diabase or alkaline lamprophyre either by partial melting or fractional crystallization from calc-alkaline lamprophyre. The parental magmatic sources of the studied dykes were generated from crustal material with addition of mantle-derived melt during the post-collisional stage. The mafic dykes in Wadi Mandar-Wadi Um Adawi area were generated from different magmatic sources by partial melting and subsequent fractional crystallization. In addition, the crustal contamination/assimilation process has a prominent role in the magmatic evolution of diabase and alkaline lamprophyre dykes.  相似文献   

5.
The Motru Dyke Swarm intrudes the Precambrian Danubian basement of the Southern Carpathians (Romania). It is a marker of a sub-volcanic event that occurred during the early Palaeozoic (Cambrian to Ordovician). The geographical distribution of dykes on a ∼2,000 km2 area is heterogeneous; several areas of high dyke density have been the subject of a detailed petrological and geochemical study. Taken altogether, the 150 samples define a single complete magmatic series, from basaltic andesite to rhyolite. Whole-rock major element variations show a medium- to high-K, calc-alkaline magmatic suite. The compositional variations and the general decrease of trace element contents (both compatible and incompatible, including REEs) from basaltic andesite to rhyolite are consistent with 1) the fractionation of the observed phenocryst assemblages, Ca-amphibole (Ti-pargasite to magnesiohornblende) followed by intermediate plagioclase, clinopyroxene and accessory biotite and quartz and 2) the absence of lower and/or upper crustal contamination. Trace elements diagrams display typical arc patterns (LILE, Pb and LREE enrichment and relative depletion in Nb-Ta, Zr-Hf and Ti). The Th/U, Nb/Ta and Zr/Hf ratios are constant and close to the mantle values throughout the whole series, which argues that the parental magma was generated from a single and homogeneous enriched lithospheric mantle source. The field regional evidence implies that melting occurred during a late- to post-orogenic period of lithospheric extension, and thus took place quite lately after the cessation of Pan-African subduction.  相似文献   

6.
The Izera Complex (West Sudetes) contains widespread bodies of metagabbro, metadolerite and amphibolite (the Izera metabasites), and less abundant dykes of weakly altered dolerites, emplaced in a continental setting. The primary magmas of the Izera metabasites were probably formed through adiabatic decompression melting of upwelling asthenosphere (mantle plume) that was associated with the early Palaeozoic fragmentation of Gondwana (initial rift). The rocks are mildly alkaline, transitional-to-tholeiitic basalts and have OIB-like trace element patterns. Trace element modelling reveals that the mafic magmas were generated by variable degrees of partial melting (1–7%) of fertile, garnet-bearing asthenospheric source similar in composition to primitive mantle. Together with an increase in degree of partial melting, the compositional affinity of the magmas and the depth of segregation changed progressively from ca. 70–90 km (mildly alkaline magmas of the metadolerites and amphibolites) to ca. 60–75 km (transitional-to-tholeiitic magmas of the metagabbros). The systematics of incompatible versus compatible element distribution, and major and trace element modelling, indicate that some rocks experienced low-pressure (<5 kbar) differentiation resulting in up to 50% fractionation of clinopyroxene, olivine and minor plagioclase and ilmenite. The genetically distinct weakly altered dolerites are basaltic andesite in composition and possibly related to late- or post-orogenic events in the Karkonosze-Izera Block. These rocks are calc-alkaline, with relatively flat MREE–HREE patterns, enrichment in LREE and other highly incompatible elements relative to primitive mantle, and negative Nb–Ta, Ti, P anomalies. The geochemical features and geochemical modelling, indicate that their primary magmas segregated at depths ≤70 km and were produced by ~2% melting of a metasomatized sublithospheric mantle source presumably containing small amounts of hydrated phases. Although the present study is inconclusive as to the origin of the metasomatic component in the source (? slab-derived fluid/melts, OIB-like alkaline melt percolation of subcontinental lithosphere), the genesis of the Izera basaltic andesites is seemingly related to upwelling of asthenosphere and heat flow triggered by a postulated decoupling of the mantle lithosphere and post-collisional extensional collapse and uplift in the Karkonosze-Izera Block.  相似文献   

7.
Numerous early Cretaceous mafic and alkaline dykes, mostly trending in N-S direction, are emplaced in the Archaean gneissic complex of the Shillong plateau, northeastern India. These dykes are spatially associated with the N-S trending deep-seated Nongchram fault and well exposed around the Swangkre-Rongmil region. The petrological and geochemical characteristics of mafic dykes from this area are presented. These mafic dykes show very sharp contact with the host rocks and do not show any signature of assimilation with them. Petrographically these mafic dykes vary from fine-grained basalt (samples from the dyke margin) to medium-grained dolerite (samples from the middle of the dyke) having very similar chemical compositions, which may be classified as basaltic-andesite/andesite. The geochemical characteristics of these mafic dykes suggest that these are genetically related to each other and probably derived from the same parental magma. Although, the high-field strength element (+rare-earth elements) compositions disallow the possibility of any crustal involvement in the genesis of these rocks, but Nb/La, La/Ta, and Ba/Ta ratios, and similarities of geochemical characteristics of present samples with the Elan Bank basalts and Rajmahal (Group II) mafic dyke samples, suggest minor contamination by assimilation with a small amount of upper crustal material. Chemistry, particularly REE, hints at an alkaline basaltic nature of melt. Trace element modelling suggests that the melt responsible for these mafic dykes had undergone extreme differentiation (∼ 50%) before its emplacement. The basaltic-andesite nature of these rocks may be attributed to this differentiation. Chemistry of these rocks also indicates ∼ 10–15% melting of the mantle source. The mafic dyke samples of the present investigation show very close geochemical similarities with the mafic rocks derived from the Kerguelen mantle plume. Perhaps the Swangkre-Rongmil mafic dykes are also derived from the Kerguelen mantle plume.  相似文献   

8.
The Shushui Complex can be divided into three rock units based on field investigation, petrography and geochemistry:(1) felsic gneisses, (2) supracrustal rocks consisting of amphibolite, marble and quartzite, and (3) late granites. Of the complex, felsic gneisses are dominant and formed in the Late Archaean, which were intruded by a basic dyke with a whole-rock Rb-Sr isochron age of 2264±219 Ma. The data on rare-earth elements as well as on major and trace elements presented for most of the rock types making up the complex suggest that (1) basic gneisses were produced by partial melting of mantle peridotite, followed by fractional crystallization, and (2) felsic gneisses produced by varying degree of melting of a mafic source. The most suitable tectonic setting to account for the generation of both types would be similar to the underplate setting.  相似文献   

9.
The petrogenesis of bronzite-picrite, olivine-gabbro, norite and quartz-tholeiite dykes, which make up the 2.39 b.y. Scourie dyke swarm cutting the Archaean Lewisian gneisses of N.W. Scotland, is interpreted on the basis of their major and trace element geochemistry. Most of the dykes bear primary amphibole and/or phlogopite and, with one exception, are all hypersthene- or quartz-normative. Apart from one tholeiite dyke which shows relative light rare-earth element depletion, all the dykes show enrichment in light rare-earths and large-ion lithophile elements. They do not however show an equivalent enrichment in other incompatible high field strength ions such as Nb and Ta, and in this respect resemble island arc and calc-alkaline basalts. The different dyke types have distinctive rare-earth patterns and other trace element ratios which are maintained over a range of major element compositions.Petrogenetic modelling of the major and trace element compositions of the various dykes demonstrates that very few can be related by fractional crystallisation. Indeed, even with partial melting mechanisms at least two different mantle sources, with different major and trace element compositons, are required to explain the compositional differences between the major dyke types. The high degrees of mantle melting implied for the generation of the magnesium-rich bronzite-picrites suggests that their rare earth and other trace element patterns closely reflect those of their mantle source. Similar arguments, though less well constrained, can be advanced for the other dyke types. The results suggest that the sub-continental mantle source feeding the dykes was heterogeneous with respect to both major and trace elements, and that their mantle sources must have been enriched in lithophile elements. Enrichment at the time the Lewisian gneisses were generated (i.e. 2.92 b.y. ago) would be compatible with the initial 87Sr/86Sr ratios of the dykes and the inferred Rb/Sr ratios of their mantle sources. The sub-continental mantle sources have thus retained the geochemical signature of the crustgenerating processes some 500 m.y. earlier.  相似文献   

10.
Approximately 1650-Ma-old NW/SE and NE/SW-trending dolerite dykes in the Tiruvannamalai (TNM) area and approximately 1800-Ma-old NW/SE-trending dolerite dykes in the Dharmapuri (DP) area constitute major Proterozoic dyke swarms in the high-grade granulite region of Tamil nadu, southern India. The NW- and NE-trending TNM dykes are compositionally very similar and can be regarded as having been formed during a single magmatic episode. The DP dykes may relate to an earlier similar magmatic episode. The dolerites are Fe-rich tholeiites and most of the elemental variations can be explained in terms of fractional crystallisation. Clinopyroxene and olivine are the inferred ferromagnesian fractionation phases followed by plagioclase during the late fractionation stages. All the studied dykes have, similar to many continental flood basalts (CFB), large-ion lithophile element (LILE) and light rare-earth element (LREE) enrichment and Nb and Ta depletion. The incompatible element abundance patterns are comparable to the patterns of many other Proterozoic dykes in India and Antarctica, to the late Archaean (～2.72 Ga) Dominion volcanics in South Africa and to the early Proterozoic (～2.0 Ga) Scourie dykes of Scotland. The geochemical characteristics of the TNM and DP dykes cannot be explained by crustal contamination alone. Instead, they are consistent with derivation from an enriched lithospheric mantle source which appears to have been developed much earlier than the dyke intrusions during a major crustal building event in the Archaean. The dyke magmas may have been formed by dehydration melting induced by decompression and lithospheric attenuation or plume impingement at the base of the lithosphere. These magmas, compared with CFB, appear to be the minor partial melts from plume heads of smaller diameter and of shallow origin (650 km). Therefore, the Proterozoic thermal events could induce crustal attenuation and dyke intrusions in contrast to the extensive CFB volcanism and continental rifting generally associated with the Phanerozoic plumes of larger head diameter (>1000 km) and of deeper origin (at crust mantle boundary).  相似文献   

11.
The basement gneiss of the Shillong plateau and the overlying rocks of the Shillong basin have been dissected by a number of mafic dyke swarms represented by older Proterozoic dolerite dykes and sills named as Khasi greenstone and the younger set of Cretaceous dykes. The older dolerites dykes of Shillong basin are partly metamorphosed and have undergone low-grade metamorphism compared to the fresh unmetamorphosed Cretaceous dykes dominantly exposed in the BGC of West Garo hills region. The Khasi greenstones are tholeiitic in composition and range from basalt to basaltic andesite. Most of the metamorphosed mafic dykes indicate continental nature with some amount of overlapping oceanic tectonic setting. Palaeomagnetic study of the metadolerite dykes show a direction of magnetization of Dm=29, Im=38 (α95 = 28.84; k = 48.33; N = 2) with a palaeolatitude of 21.3° N to the Indian sub-continent that clearly support the Proterozoic dyke/dyke swarm emplacement in the region. The magnetic carrier as inferred from IRM studies is magnetite in the range of psuedosingle to multi domain (MD) states with minor contribution from hematite.  相似文献   

12.
The Indian Shield is cross-cut by a number of distinct Paleoproterozoic mafic dyke swarms. The density of dykes in the Dharwar and Bastar Cratons is amongst the highest on Earth. Globally, boninitic dyke swarms are rare compared to tholeiitic dyke swarms and yet they are common within the Southern Indian Shield. Geochronology and geochemistry are used to constrain the petrogenesis and relationship of the boninitic dykes (SiO2 = 51.5 to 55.7 wt%, MgO = 5.8 to 18.7 wt%, and TiO2 = 0.30 wt% to 0.77 wt%) from the central Bastar Craton (Bhanupratappur) and the NE Dharwar Craton (Karimnagar). A single U-Pb baddeleyite age from a boninitic dyke near Bhanupratappur yielded a weighted-mean 207Pb/206Pb age of 2365.6 ± 0.9 Ma that is within error of boninitic dykes from the Dharwar Craton near Karimnagar (2368.5 ± 2.6 Ma) and farther south near Bangalore (2365.4 ± 1.0 Ma to 2368.6 ± 1.3 Ma). Rhyolite-MELTS modeling indicates that fractional crystallization is the likely cause of major element variability of the boninitic dykes from Bhanupratappur whereas trace element modeling indicates that the primary melt may be derived from a pyroxenite mantle source near the spinel-garnet transition zone. The Nd isotopes (εNd(t) = −6.4 to +4.5) of the Bhanupratappur dykes are more variable than the Karimnagar dykes (εNd(t) = −0.7 to +0.6) but they overlap. The variability of Sr-Nd isotopes may be related to crustal contamination during emplacement or is indicative of an isotopically heterogeneous mantle source. The chemical and temporal similarities of the Bhanupratappur dykes with the dykes of the Dharwar Craton (Karimnagar, Penukonda, Chennekottapalle) indicate they are members of the same giant radiating dyke swarm. Moreover, our results suggest that the Bastar and Dharwar Cratons were adjacent but likely had a different configuration at 2.37 Ga than the present day. It is possible that the 2.37Ga dyke swarm was related to a mantle plume that assisted in the break-up of an unknown or poorly constrained supercontinent.  相似文献   

13.
华北克拉通1.75Ga基性岩墙群特征及其研究进展   总被引：2，自引：1，他引：2  
基性岩墙群是地壳伸展背景下,来自地幔的基性岩浆侵入体。华北克拉通同世界上其它克拉通一样,广泛发育前寒武纪基性岩墙群。它们在不同时代均有产生,其中1.75Ga前后的规模最大,分布范围最广,几乎遍布整个克拉通,对其进行深入研究,可以揭示华北克拉通该期构造演化过程。华北克拉通1.75Ga前后的岩墙几何形态多变,直立或近直立,走向主要为NNW向和近EW向。岩石以拉斑玄武质岩类占绝对优势(>80%),主要造岩矿物为单斜辉石和斜长石。根据岩墙走向、岩浆分异程度和岩石地球化学特征可将其分五组:低分异LT组、低分异HT组、高分异NW组、高分异EW组,以及具明显差异的高铁系列。同位素和微量元素研究显示,岩浆源区主要与富集Ⅰ型地幔(EMⅠ)、弱亏损的常规地幔(DM-PREMA)以及陆下岩石圈地幔有关。目前对华北克拉通1.75Ga基性岩墙群产出的构造环境在认识上有分歧,其中地幔柱观点和碰撞后伸展观点最为人们所关注。  相似文献   

14.
Mafic dykes intrude the composite Mt. Abu granite batholith as a minor and the last phase of magmatism. The dykes are sub-vertical, variable in width and visibly compact, however, features of alteration and shearing can be seen. The dykes occurring within the recently identified and described, Delwara Shear Zone (DWSZ), from the western margin of the Mt. Abu batholith are intensely to moderately sheared and intricately mixed with the host granitoids. The mafic dykes occurring within the shear zone bear evidence of assimilating the host granitoids during their ascent, seen as relicts, streaks and sub-rounded K-feldspar clasts in mafic dykes. The hybridization has resulted in unusual geochemical signatures of the mafic dykes such as higher silica levels, erratic and high incompatible trace element abundances and lack of any systematic trends. Mixing line calculations on the mafic dyke samples reveal between 30 to 60% felsic input into the mafic dykes. Mafic dykes outside the shear zone in the Mt. Abu are meter scale in width and generally free of felsic inclusions owing to small volumes of mafic melts. Large volume of mafic melts are required for assimilating up to 60% felsic component which has been identified as approximately 100 m wide zone within the DWSZ. Shearing has played an important role in providing the channel ways and for sustained high temperatures to allow such hybridization.  相似文献   

15.
骆文娟  张招崇  侯通  王萌 《岩石学报》2011,27(10):2947-2962
茨达复式岩体位于中国西南扬子地台西缘的攀西裂谷内,其岩性从基性到酸性连续变化,SiO2含量为40.06％ ～68.54％,但以基性和酸性岩石为主,中性岩石较少,而且非常不均匀,通常具有斑杂构造特征.从基性岩到酸性岩,各岩石样品由轻稀土弱富集型变为较强富集型.微量元素表现为酸性岩中Rb、Th、K、La、Ce、Pb、Nd、Zr、Hf、Sm呈正异常和Ba、Nb、Ta、Sr、P、Ti的负异常;基性岩除Ti负异常和Pb正异常外,其它异常不明显;中性岩具有Ti、Sr负异常和Pb正异常,其它特征介于基性岩和酸性岩石之间.野外和岩相学特征明显指示出中性岩石具有混合特征.酸性端元岩浆准铝质的特征以及相对低的SiO2含量指示其起源于玄武质下地壳的部分熔融,而基性端元岩浆的地球化学特征以及高温特征暗示着其起源于地幔柱源区.锆石U-Pb年龄数据表明,该复式岩体中基性端元LA-MC-ICP-MS U-Pb锆石年龄为243.76±0.77Ma,酸性端元年龄为240.5±0.76Ma,可能代表了峨眉山大火成岩省岩浆活动的尾声阶段.  相似文献   

16.
Singhbhum granitoid complex has been intruded by numerous mafic dykes trending in different directions. The studied dykes were originated from subalkaline magma, ranging in composition from basalt through basaltic-andesite to andesite. In the present work, the studied dykes are divided into two groups on the bases of abundance of SiO2, MgO, Al2O3, Ni, and Cr. The first one has higher Mg# than that of group II. These dykes have enriched incompatible trace element patterns. These are particularly enriched in the light rare earth elements and large ion lithophile elements with depleted high field strength elements (Nb, P, Ti). High Ba/Nb and Sr/P ratios of present mafic dykes are the indications of subduction signature. The geochemical characters of the back arc extension basalt tectonic setting is suggested for the studied dykes. Higher PM-normalized Th/Nb, Rb/Nb, and Ba/Nb ratios of studied samples support their non-plume source.  相似文献   

17.
The Archaean gneissic basement of Shillong plateau has been traversed by number of mafic dyke swarms. At least two suites of dykes are identified in the region represented by Proterozoic Khasi greenstone related dolerites and younger Cretaceous dolerite dykes in addition to mafic alkaline dykes. The older Khasi greenstone dolerites are altered and have undergone low-grade metamorphism compared to fresh Cretaceous dykes, which are well exposed in the West Garo Hills region. All the Khasi greenstone dolerites are tholeiite in composition and range from basalt to basaltic andesite in composition and show olivine or quartz normative character. Most of the dykes show continental nature of emplacement with some overlapping oceanic tectonic setting of origin. Petrochemical study suggests that they were derived from picrites that subsequently undergone low-pressure fractionation. Palaeomagnetic study of the older Khasi greenstone related dolerites show a direction of magnetization of Dm=17, Im= +57 (α95= 23.34; K=31.5; N=24) with a palaeolatitude of 29.7°N to the Indian subcontinent that clearly support the Proterozoic dyke/dyke swarm emplacement in the region. The magnetic carrier as inferred from K-T studies is in multi domain (MD) size and cation deficient (CD) domain states.  相似文献   

18.
胡国辉  胡俊良  陈伟  赵太平 《岩石学报》2010,26(5):1563-1576
幔源岩浆侵位产生的基性岩墙群是地壳伸展裂解的重要标志。华北克拉通南缘中条山-嵩山地区出露有大量的中元古代基性岩墙群,对其研究将有助于深入了解华北克拉通中元古代构造演化特征。其岩石类型以辉绿岩为主,少量辉长辉绿岩和辉绿玢岩;主要造岩矿物为斜长石和单斜辉石,其它矿物包括角闪石、Fe-Ti氧化物、磷灰石、黑云母、碱性长石和石英。锆石SHRIMPU-Pb年龄为1785±18Ma,代表岩墙的结晶年龄。岩石K2O+Na2O含量为3.63%～6.18%,K2O/Na2O比值为0.73～1.38,FeOT含量较高(10.03%～13.59%),属于拉斑玄武岩系列。岩石的稀土元素含量高(142×10-6～381×10-6),亏损Nb、Ta、Zr、Hf等高场强元素,富集Rb、Ba、La等大离子亲石元素,全岩εNd(t)值为-6.2～-8.1。岩墙的固结指数(SI)和MgO含量呈明显的正相关关系,说明幔源岩浆发生过明显的结晶分异作用。地球化学分析表明,研究区基性岩墙属于板内拉斑玄武岩系列,与熊耳群火山岩有相似的地球化学特征;基于两者紧密的产出关系,我们认为该区岩墙可能是熊耳群火山岩的通道岩墙,共同代表了华北克拉通东、西陆块碰撞后伸展事件。  相似文献   

19.
古元古代是华北克拉通形成过程中重要的造山构造演化阶段,该阶段形成的基性岩墙群,为深入理解裂解-俯冲-碰撞-抬升的造山构造-岩浆过程提供了重要信息.本文报道了天镇-怀安地区广泛分布于新太古代-古元古代变质基底中的变质基性岩墙（二辉麻粒岩）,野外产状与区域主期构造面理协调一致,主要由单斜辉石、斜方辉石、斜长石和少量角闪石组成.LA-MC-ICPMS锆石U-Pb同位素定年获得变质基性岩墙的变质年龄为1 820~1 834 Ma,与区内麻粒岩相变质事件一致,结合区域基性岩墙年龄记录,推测其原岩形成年龄为1.95~1.91 Ga.根据岩石地球化学特征可将变质基性岩墙划分为高Mg低Ti型和低Mg高Ti型两类,两者经历了不同程度的橄榄石、单斜辉石和斜长石的分离结晶.两类基性岩墙均亏损高场强元素（如Nb、Ta、Ti、Zr和Hf）,结合锆石Hf同位素分析,研究表明基性岩墙来源于俯冲流体交代的岩石圈地幔或者受到过地壳物质的混染.华北克拉通古元古代存在2.16~2.04 Ga和1.97~1.83 Ga两期基性岩墙侵位事件:早期代表在初始克拉通基础上发生的板内裂解过程,晚期记录了由俯冲碰撞到伸展的转换过程,即碰撞造山构造体制由水平挤压转变为垂向抬升,构造转换时限大致介于1.95~1.91 Ga.   相似文献   

20.
High- and Low-Temperature I-type Granites   总被引：4，自引：0，他引：4  
Abstract: I– and S-type granites differ in several distinctive ways, as a consequence of their derivation from contrasting source rocks. The more mafic granites, whose compositions are closest to those of the source rocks, are most readily classified as I– or S–type. As granites become more felsic, compositions of the two types converge towards those of lowest temperature silicate melts. While discrimination of the two is therefore more difficult for such felsic rocks, that in no way invalidates the twofold subdivision. If felsic granite melts undergo fractional crystallisation, the major element compositions are not affected to any significant extent, but the concentrations of trace elements can vary widely. For some trace elements, fractional crystallisation causes the trace element abundances to diverge, so the I– and S– type granites are again easily separated. Such fractionated S-type granites can be distinguished, for example, by high P and low Th and Ce, relative to their I-type analogues. Our observations in the Lachlan Fold Belt show that there is no genetic basis for subdividing peraluminous granites into more mafic and felsic varieties, as has been attempted elsewhere. The subdivision of felsic peraluminous granites into I– and S-types is more appropriate, and mafic peraluminous granites are always S–type. In a given area, associated mafic and felsic S-type granites are likely to be closely related in origin, with the former comprising both restite-rich magmas and cumulate rocks, and the felsic granites corresponding to melts that may have undergone fractional crystallisation after prior restite separation. We propose a subdivision of I-type granites into two groups, formed at high and low temperatures. The high-temperature I–type granites formed from a magma that was completely or largely molten, and in which crystals of zircon were not initially present because the melt was undersaturated in zircon. In comparison with low-temperature I–type granites, the compositions extend to lower SiO2 contents and the abundances of Ba, Zr and the rare earth elements initially increase with increasing SiO2 in the more mafic rocks. While the high-temperature I–type granite magmas were produced by the partial melting of mafic source rocks, their low-temperature analogues resulted from the partial melting of quartzofeldspathic rocks such as older tonalites. In that second case, the melt produced was felsic and the more mafic low-temperature I–type granites have that character because of the presence of entrained and magmatically equilibrated restite. High temperature granites are more prospective for mineralisation, both because of that higher temperature and because they have a greater capacity to undergo extended fractional crystallisation, with consequent concentration of incompatible components, including H2O.  相似文献   

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