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1.
The varying geochemical and petrogenetic nature of A-type granites is a controversial issue. The oxidized, magnetite-series A-type granites, defined by Anderson and Bender [Anderson, J.L., Bender, E.E., 1989. Nature and origin of Proterozoic A-type granitic magmatism in the southwestern United States of America. Lithos 23, 19–52.], are the most problematic as they do not strictly follow the original definition of A-type granites, and approach calc-alkaline and I-type granites in some aspects. The oxidized Jamon suite A-type granites of the Carajás province of the Amazonian craton are compared with the magnetite-series granites of Laurentia, and other representative A-type granites, including Finnish rapakivi and Lachlan Fold Belt A-type granites, as well as with calc-alkaline, I-type orogenic granites. The geochemistry and petrogenesis of different groups of A-types granites are discussed with an emphasis on oxidized A-type granites in order to define their geochemical signatures and to clarify the processes involved in their petrogenesis. Oxidized A-type granites are clearly distinguished from calc-alkaline Cordilleran granites not only regarding trace element composition, as previously demonstrated, but also in their major element geochemistry. Oxidized A-type granites have high whole-rock FeO t/(FeO t + MgO), TiO 2/MgO, and K 2O/Na 2O and low Al 2O 3 and CaO compared to calc-alkaline granites. The contrast of Al 2O 3 contents in these two granite groups is remarkable. The CaO/(FeO t + MgO + TiO 2) vs. CaO + Al 2O 3 and CaO/(FeO t + MgO + TiO 2) vs. Al 2O 3 diagrams are proposed to distinguish A-type and calc-alkaline granites. Whole-rock FeO t/(FeO t + MgO) and the FeO t/(FeO t + MgO) vs. Al 2O 3 and FeO t/(FeO t + MgO) vs. Al 2O 3/(K 2O/Na 2O) diagrams are suggested for discrimination of oxidized and reduced A-type granites. Experimental data indicate that, besides pressure, the nature of A-type granites is dependent of ƒO 2 conditions and the water content of magma sources. Oxidized A-type magmas are considered to be derived from melts with appreciable water contents (≥ 4 wt.%), originating from lower crustal quartz-feldspathic igneous sources under oxidizing conditions, and which had clinopyroxene as an important residual phase. Reduced A-type granites may be derived from quartz-feldspathic igneous sources with a metasedimentary component or, alternatively, from differentiated tholeiitic sources. The imprint of the different magma sources is largely responsible for the geochemical and petrological contrasts between distinct A-type granite groups. Assuming conditions near the NNO buffer as a minimum for oxidized granites, magnetite-bearing granites formed near FMQ buffer conditions are not stricto sensu oxidized granites and a correspondence between oxidized and reduced A-type granites and, respectively, magnetite-series and ilmenite-series granites is not always observed. 相似文献
2.
Many continental flood basalts (CFB) have isotope and trace-element signatures that differ from those of oceanic basalts and much interest concerns the extent to which these reflect differences in their upper mantle source regions. A review of selected data sets from the Mesozoic and Tertiary CFB confirms significant differences in their major- and trace-element compositions compared with those of basalts erupted through oceanic lithosphere. In general, those CFB suites characterised by low Nb/La, high ( 87Sr/ 86Sr) i and low εNd i tend to exhibit relatively low TiO 2, CaO/Al 2O 3, Na 2O and/or Fe 2O 3, and relatively high SiO 2. In contrast, those which have high Nb/La, low ( 87Sr/ 86Sr) i and high εNd i ratios, like the upper units in the Deccan Traps, have major- and trace-element compositions similar to oceanic basalts. It would appear that those CFB that have distinctive isotope and trace-element ratios also exhibit distinctive major-element contents, suggesting that major and trace elements have not been decoupled significantly during magma generation and differentiation. When compared (at 8% MgO) with oceanic basalt trends, the displacement of many CFB to lower Na2O, Fe2O3*, TiO2 and CaO/Al2O3, but higher SiO2, at similar Mg#, is not readily explicable by crustal contamination. Rather, it reflects source composition and/or the effects of the melting processes. The model compositions of melts produced by decompression of mantle plumes beneath continental lithosphere have relatively low SiO2 and high Fe2O3*. In contrast, the available experimental data indicate that partial melts of peridotite have low TiO2, Na2O and Fe2O3*CaO/Al2O3, if the peridotite has been previously depleted by melt extraction. Moreover, melting of hydrated, depleted peridotite yields SiO2-rich, Fe2O3- and CaO-poor melts. Since anhydrous, depleted peridotite has a high-temperature solidus, it is argued that the source of these CFB was variably melt depleted and hydrated mantle, inferred to be within the lithosphere. Isotope data suggest these source regions were often old and relatively enriched in incompatible trace elements, and it is envisaged that H2O±CO2 were added at the same time as the incompatible elements. An implication is that a significant proportion of the new continental crust generated since the Permian reflected multistage processes involving mobilization of continental mantle lithosphere that was enriched in minor and trace elements during the Proterozoic. 相似文献
3.
通过对出露于西藏南部岗巴—定日地区花岗岩体的地球化学研究表明,岩石中SiO2,Al2O3,Na2O和FeO,MgO等的含量均高,贫CaO和Fe2O3;w(SiO2)介于71.40%～73.06%,A/CNK在1.17～1.34之间,为铝和硅过饱和类型的强过铝质花岗岩。岩石的稀土元素总量(ΣREE)为56.80×10-6～89.12×10-6,(La/Yb)N=6.30～18.26,(La/Sm)N=2.62～3.40,ΣLREE/ΣHREE=2.41～4.66;稀土元素配分曲线呈右倾型,具有弱的负铕异常。Nb,Ti等高场强元素具有明显的亏损,而Rb,U,La,Nd,Hf,Eu,Y等大离子亲石元素具有明显的正异常。岩石的87Sr/86Sr初始比值较高,87Sr/86Sr为(0.738 71～0.751 12)。综合研究认为,本区花岗岩的成因为陆壳部分熔融作用形成的,属陆壳改造型强过铝质花岗岩。本区花岗岩岩浆源区岩石成分主要为砂屑岩,其次为泥质岩,是上地壳部分熔融作用的结果。岩石的微量元素标准化曲线图、岩石地R1-R2图解、Rb-(Yb+Ta)和Rb-(Nb+Yb)图解均显示本区岩体形成于同碰撞构造环境的花岗岩,具有同碰撞岩浆活动的特征,是喜马拉雅早期印度板块与冈底斯板块的俯冲碰撞导致的地壳增厚,上地壳部分熔融的产物;为形成于同碰撞构造环境的花岗岩。 相似文献
4.
The Komati Formation of the Barberton greenstone belt (BGB), South Africa, is composed of both Al-undepleted and -depleted komatiites. The Al-undepleted komatiites are characterised by Al 2O 3/TiO 2 and CaO/Al 2O 3 ratios of 15–18 and 1.1–1.5, respectively, and exhibit chondritic trace element contents and (Gd/Yb) N ratios. In contrast, the Al-depleted komatiites show significantly lower Al 2O 3/TiO 2 ratios of 8–12, highly variable CaO/Al 2O 3 (0.19–2.81) ratios combined with (Gd/Yb) N ratios varying from 1.08 to 1.56. A Sm–Nd whole rock isochron for komatiites of the Komati Formation gives an age of 3657±170 Ma. 147Sm/ 144Nd ratios (0.1704 and 0.1964) are all lower than the chondritic value of 0.1967. The komatiite i,Nd(3.45) values cluster at +1.9±0.7. Trace element distribution indicates that most of the primary geochemical and isotopic features of the komatiites were preserved in line with the conservation of the primary chemical composition of clinopyroxene. High field strength element and rare earth element abundances indicate that crustal contamination and post-crystallisation processes did not disturb the primary features of komatiites. The Sm/Nd and Nb/U ratios of komatiites indicate that the Barberton greenstone belt mantle source has undergone melt extraction prior to komatiite formation. Variations of Al2O3/TiO2, (Gd/Yb)N, Zr/Sm and Sm/Nd ratios of komatiites indicate that a batch melting of slightly depleted mantle source during with garnet and/or clinopyroxene remained in the residue can produce the geochemical isotopic feature of the Barberton greenstone belt komatiites. Typical geochemical fingerprints of subduction-related processes (LILE enrichment, HFSE depletion compared to REE), as known from modern subduction zones, are not observed. Komatiites exhibit Ti/Zr, La/Nb, Nb/U, Sr/Nd and Ba/La ratios comparable to those of oceanic island basalt and mid-ocean ridge basalt. (La/Nb)PMN, (Sm/Yb)PMN, positive δNb values and flat or slightly enriched REE patterns suggest that BGB komatiites are part of an oceanic plateau rather than an oceanic island such as Iceland. Therefore, an oceanic plateau or mid-ocean ridge, in connection with an oceanic plateau, such as Ontong Java plateau or Caribbean–Colombian oceanic plateau, is a suitable tectonic setting for the formation of the BGB komatiites. 相似文献
5.
The main anatectic granite of the Velay complex is unique among major French Massif Central Hercynian granitoids in that rather than having an entirely lower crustal source, it formed by mixing between partial melts of the meta-igneous lower crust and ‘upper crustal’ country rock schists and orthogneisses. The geochemical variations in the Velay main anatectic granites cannot, however, be explained by mixing alone as their compositions range to lower SiO 2, with higher Al 2O 3, Fe 2O 3 and TiO 2 and lower Na 2O and CaO, than either end member in mixing. The variations are interpreted as being due to the presence of up to 35% restite in minimum melts of country rock compositions. Primary restites form equilibrium assemblages represented by biotite, ilmenite and surmicaceous enclaves which consist of biotite ± apatite, zircon and almandine. The main anatectic granites more rarely contain schist and gneiss enclaves, quartz resisters and plagioclase restites. Secondary restites are mainly represented by cordierite, and possibly K-feldspar, which formed by recrystallisation of primary biotite-rich restites. The unique characteristics of the Velay main anatectic granites are likely to be due, in part, to its late formation close to the end of the Hercynian orogeny. The metasedimentary lower crust may have become too refractory to yield large volumes of melt following partial melting to form the other major Massif Central granitoids. The heat necessary for partial melting at higher crustal levels was transferred from the lower crust by the intrusion of I-type granites and low volume diorites from the mantle. Upper crustal anatexis was mainly controlled by muscovite breakdown reactions (< 830 to 850 °C) and the liberation of water due to the recrystallisation of biotite to cordierite. The temperatures necessary for biotite breakdown were only achieved locally and resulted in the formation of high-LREE granites. 相似文献
6.
At least 14 small (1–11 km across) 1.8 Ga Svecofennian post-collisional bimodal intrusions occur in southern Finland and Russian Karelia in a 600-km-long belt from the Åland Islands to the NW Lake Ladoga region. The rocks range from ultramafic, calc-alkaline, apatite-rich potassium lamprophyres to peraluminous HiBaSr granites, and form a shoshonitic series with K 2O+Na 2O>5%, K 2O/Na 2O>0.5, Al 2O 3>9% over a wide spectrum of SiO 2 (32–78%). Although strongly enriched in all rocks, the LILE Ba and Sr and the LREE generally define a decreasing trend with increasing SiO 2. Depletion is noted for HFSE Ti, Nb and Ta. Available isotopic data show overlapping values for lamprophyres and granites within separate intrusions and a cogenetic origin is thus not precluded. Initial magmas (Mg#>65) in this shoshonitic association are considered to be generated in an enriched lithospheric mantle during post-collisional uplift some 30 Ma after the regional Svecofennian metamorphic peak. However, prior to the melting episode, the lithospheric mantle was affected by carbonatite metasomatism; more extensively in the east than in the west. The melts generated in the more carbonate-rich mantle are extremely enriched in P 2O 54%, F12,000 ppm, LILE: Ba9000 ppm, Sr7000 ppm, LREE: La600 ppm and Ce1000 ppm. The parental magma underwent 55–60% fractionation of biotite+clinopyroxene+apatite+magnetite+sphene whereupon intermediate varieties were produced. After further fractionation, 60–80%, of K-feldspar+amphibole+plagioclase±(minor magnetite, sphene and apatite), leucosyenites and quartz-monzonites were formed. In the west, where the source was less affected by carbonatite metasomatism, calc-alkaline lamprophyres (vogesites, minettes and spessartites) and equivalent plutonic rocks (monzonites) were formed. Removal of about 50% of biotite, amphibole, plagioclase, magnetite, apatite and sphene produced peraluminous HiBaSr granites. The impact of crustal assimilation is considered to be low. At about 1.8 Ga, the post-collisional shoshonitic magmatism brought juvenile material, particularly enriched in alkalis, LILE, LREE and F, into the crust. Although areally restricted, the regional distribution of the post-collisional intrusions may indicate that larger volumes of 1.8 Ga juvenile material resides in unexposed parts of the crust. 相似文献
7.
Radiogenic isotope data (initial Nd, Pb) and elemental concentrations for the Mooselookmeguntic igneous complex, a suite of mainly granitic intrusions in New Hampshire and western Maine, are used to evaluate petrogenesis and crustal variations across a mid-Paleozoic suture zone. The complex comprises an areally subordinate monzodiorite suite [377±2 Ma; εNd (at 370 Ma)=−2.7 to −0.7; initial 207Pb/ 204Pb=15.56–15.58] and an areally dominant granite [370±2 Ma; εNd (at 370 Ma)=−7.0 to −0.6; initial 207Pb/ 204Pb=15.55–15.63]. The granite contains meter-scale enclaves of monzodiorite, petrographically similar to but older than that of the rest of the complex [389±2 Ma; εNd (at 370 Ma)=−2.6 to +0.3; initial 207Pb/ 204Pb 15.58, with one exception]. Other granite complexes in western Maine and New Hampshire are 30 Ma older than the Mooselookmeguntic igneous complex granite, but possess similar isotopic signatures. Derivation of the monzodioritic rocks of the Mooselookmeguntic igneous complex most likely occurred by melting of Bronson Hill belt crust of mafic to intermediate composition. The Mooselookmeguntic igneous complex granites show limited correlation of isotopic variations with elemental concentrations, precluding any significant presence of mafic source components. Given overlap of initial Nd and Pb isotopic compositions with data for Central Maine belt metasedimentary rocks, the isotopic heterogeneity of the granites may have been produced by melting of rocks in this crustal package or through a mixture of metasedimentary rocks with magmas derived from Bronson Hill belt crust. New data from other granites in western Maine include Pb isotope data for the Phillips pluton, which permit a previous interpretation that leucogranites were derived from melting heterogeneous metasedimentary rocks of the Central Maine belt, but suggest that granodiorites were extracted from sources more similar to Bronson Hill belt crust. Data for the Redington pluton are best satisfied by generation from sources in either the Bronson Hill belt or Laurentian basement. Based on these data, we infer that Bronson Hill belt crust was more extensive beneath the Central Maine belt than previously recognized and that mafic melts from the mantle were not important to genesis of Devonian granite magma. 相似文献
8.
High-calcium, nepheline-normative ankaramitic basalts (MgO > 10 wt.%, CaO/Al 2O 3 > 1) from Rinjani volcano, Lombok (Sunda arc, Indonesia) contain phenocrysts of clinopyroxene and olivine (Fo 85–92) with inclusions of spinel (Cr# 58–77) and crystallised melt. Olivine crystals have variable but on average low NiO (0.10–0.23 wt.%) and high CaO (0.22–0.35 wt.%) contents for their forsterite number. The CaO content of Fo 89–91 olivine is negatively correlated with the Al 2O 3 content of enclosed spinel (9–15 wt.%) and positively correlated with the CaO/Al 2O 3 ratios of melt inclusions (0.9–1.5). Major and trace element patterns of melt inclusions are similar to that of the host rock, indicating that the magma could have formed by accumulation of small batches of melt, with compositions similar to the melt inclusions. The liquidus temperature of the magma was 1275 °C, and its oxygen fugacity ≤ FMQ + 2.5. Correlations between K 2O, Zr, Th and LREE in the melt inclusions are interpreted to reflect variable degrees of melting of the source; correlations between Al 2O 3, Na 2O, Y and HREE are influenced by variations in the mineralogy of the source. The melts probably formed from a water-poor, clinopyroxene-rich mantle source. 相似文献
9.
位于中天山地块南缘大黑山地区的阿拉塔格花岗岩体,岩性主要由花岗闪长岩、二长花岗岩、似斑状花岗岩组成,岩石具有高硅( w（SiO 2）为66.29%~77.47%)、富碱( w（Na 2O+K 2O）为6.75%~9.93%)、高铝( w（Al 2O 3）为10.97%~14.40%)、低Sr( w（Sr）为（28.78~153.00）×10 -6,平均为99.23×10 -6)、低Ti（ w（TiO 2）为0.09％~0.77％）的特征。岩石的A/CNK值为1.19~1.50,为钙碱性过铝质岩石;岩石Eu亏损（ δEu＝0.19~0.51）、LREE富集(LREE/HREE= 6.80~8.45,(La/Yb) N= 6.06~9.03),明显富集Rb、Th、K、Hf(Zr) 等大离子亲石元素(LILE),亏损Nb、Ta、P、Ti等高场强元素(HFSE);岩石的Ba含量较低,并具有明显的Sr负异常。结合区域地质特征,通过岩石的地球化学和Sr、Nd同位素综合分析,认为该花岗岩形成于后碰撞环境,且为壳幔混源的岩浆多期次侵位的复合岩体。 相似文献
10.
位于扬子板块西北缘宁强地区的大安花岗岩体,岩石类型主要为黑云母花岗闪长岩,但其形成时代却有一定的争议,成因及地质意义尚未明确.对大安花岗岩体进行详细的LA-ICP-MS锆石U-Pb年代学和岩石地球化学研究.结果表明,花岗闪长岩年龄为212.3±1.6 Ma和212.48±0.43 Ma,属晚三叠世.地球化学特征显示花岗闪长岩相对高硅（67.61%~69.02%）、高Al 2O 3（16.14%~16.80%）,Na 2O > K 2O,富集大离子亲石元素（Cs、Ba）和轻稀土元素,Eu负异常不明显,强烈富集Sr（538×10-6~907×10-6）和亏损Y（3.10×10-6~3.90×10-6）,高Sr/Y比值（138~291）,表现出明显的埃达克质岩石的地球化学特征.综合区域地质资料认为,大安花岗岩体形成于后碰撞构造环境,是在华北板块与扬子板块碰撞后期伸展体制下,由于地幔物质上涌带来的热量导致加厚基性下地壳脱水熔融,形成了具有埃达克质性质的岩浆. 相似文献
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