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1.
Richard J. Walker G. N. Hanson J. J. Papike J. R. O''Neil 《Geochimica et cosmochimica acta》1986,50(12):2833-2846
The Nd, O and Sr isotopic characteristics of Precambrian metasedimentary, metavolcanic and granitic rocks from the Black Hills of South Dakota are examined. Two late-Archean granites (2.5-2.6 Ga) have Tdm ages of 3.05 and 3.30 Ga, suggesting that at least one of the granites was derived through the melting of significantly older crust. Early-Proterozoic metasedimentary rocks have Tdm ages that range from 2.32 to 2.45 Ga. These model ages, in conjunction with probable stratigraphic ages ranging from 1.9 to 2.2 Ga, indicate that mantle-derived material was added to the continental crust of this region during the early-Proterozoic. Previous studies of the Harney Peak Granite complex have reported U-Pb and Rb-Sr ages of about 1.71 Ga and most granite samples examined in this study have Sr isotopic compositions consistent with that age. Two granite samples taken from the same sill, however, give two-point Rb-Sr and Sm-Nd ages of 2.08 ±0.08 and 2.20 ±0.20 Ga (∑2200Nd = −15.5), respectively. In addition, whole-rock and apatite samples of the spatially associated Tin Mountain pegmatite give a Sm-Nd isochron age of 2000 ±100 Ma (∑2200Nd = −5.8 ±1.8).
The Sm-Nd, O and Rb-Sr isotopic systematics of these granitic rocks have been complicated to some degree by both crystallization and post-crystallization processes, and the age of the pegmatite and parts of the Harney Peak Granite complex remain uncertain. Processes that probably complicated the isotopic systematics of these rocks include derivation from heterogeneous source material, assimilation, mixing of REE between granite and country rock during crystallization via a fluid phase and post-crystallization mobility of Sr. The Nd isotopic compositions of the pegmatite and the Harney Peak Granite indicate that they were not derived primarily from the exposed metasedimentary rocks. 相似文献
2.
Richard J. Walker Gilbert N. Hanson James J. Papike 《Contributions to Mineralogy and Petrology》1989,101(3):290-300
The Tin Mountain pegmatite is a small, zoned granitic body that is extremely enriched in Rb and Li, but has moderate concentrations of Sr and Ba. These trace elements are modelled using granitic distribution coefficients in order to test the potentials of partial melting of metasedimentary rocks and fractionation of a less-evolved granitic melt to have produced the parental liquid to the Tin Mountain pegmatite. Batch melting of any reasonable metasedimentary source rock would likely have yielded melts that were either insufficiently enriched in Rb and Li to be the parental liquid, or that had Sr and Ba concentrations that were much higher than those estimated for the parental liquid. The modelling of simple fractional crystallization and equilibrium crystallization of a granitic melt within the compositional range of the spatially associated Harney Peak Granite gives calculated melt compositions with either lower Sr and Ba concentrations or inadequate Rb and Li enrichments, to be the parent liquid of the pegmatite. At least two variants from simple crystal-liquid fractionation models can, however, successfully account for the derivation of the parent liquid: 1) generation of a Rb-, Li-, Ba- and Sr-rich granitic melt (outside of the compositional range of the sampled portions of the Harney Peak Granite complex) by low degrees of partial melting of metasedimentary rocks found in the Black Hills, followed by moderate extents of fractional or equilibrium crystallization, 2) derivation from Harney Peak granite via a complex, multi-stage crystal-liquid fractionation process, such as progressive equilibrium crystallization. 相似文献
3.
New trace element data were obtained by ICP-MS for 58 samples representing eight intrusive phases of the Raumid granite Pluton. All of the rocks, except for one sample that was deliberately taken from a greisenized zone, were not affected by postmagmatic fluid alteration. The sequential accumulation of incompatible trace elements (Rb, Ta, Nb, Pb, U, and others) in the Raumid Pluton from the early to late phases coupled with a decrease in incompatible element contents (Sr, Eu, Ba, and others) indicates a genetic link between the granites of all phases via fractional crystallization of a granite melt. The REE distribution patterns of final granite phases are typical of rare-metal granites. The Ta content in the granites of phase 8 is only slightly lower than that of typical rare-metal granites. Greisenization disturbed the systematic variations in trace element distribution formed during the magmatic stage. The ranges of trace element contents (Rb, Sr, Ta, Nb, and others) and ratios (Rb/Sr, La/Lu, Eu/Eu*, and others) in the Raumid granite overlap almost entirely the ranges of granitic rocks of various compositions, from the least differentiated with ordinary trace element contents to rare-metal granites. This indicates that the geochemical signature of rare-metal granites can develop at the magmatic stage owing to fractional crystallization of melts, which is the case for the melt of the Raumid granite. 相似文献
4.
The trace element compositions of S-type granites: evidence for disequilibrium melting and accessory phase entrainment in the source 总被引：4，自引：2，他引：2
Arnaud Villaros Gary Stevens Jean-François Moyen Ian S. Buick 《Contributions to Mineralogy and Petrology》2009,158(4):543-561
Within individual plutons, the trace element concentrations in S-type granites generally increase with maficity (total iron
and magnesium content and expressed as atomic Fe + Mg in this study); the degree of variability in trace element concentration
also expands markedly with the same parameter. The strongly peraluminous, high-level S-type granites of the Peninsular Pluton
(Cape Granite Suite, South Africa) are the product of biotite incongruent melting of a metasedimentary source near the base
of the crust. Leucogranites within the suite represent close to pure melts from the anatectic source and more mafic varieties
represent mixtures of melt and peritectic garnet and ilmenite. Trace elements such as Rb, Ba, Sr and Eu, that are concentrated
in reactant minerals in the melting process, show considerable scatter within the granites. This is interpreted to reflect
compositional variation in the source. In contrast, elements such as LREE, Zr and Hf, which are concentrated within refractory
accessory phases (zircon and monazite), show well-defined negative correlations with increasing SiO2 and increase linearly with increasing maficity. This is interpreted to reflect coupled co-entrainment of accessory minerals
and peritectic phases to the melt: leucocratic rocks cannot have evolved from the more mafic compositions in the suite by
a process of fractional crystallisation because in this case they would have inherited the zircon-saturated character of this
hypothetical earlier magma. Trace element behaviour of granites from the Peninsular Pluton has been modelled via both equilibrium
and disequilibrium trace element melting. In the disequilibrium case, melts are modelled as leaving the source with variable
proportions of entrained peritectic phases and accessory minerals, but before the melt has dissolved any accessory minerals.
Thus, the trace element signature of the melt is largely inherited from the reactants in the melting reaction, with no contribution
from zircon and monazite dissolution. In the equilibrium case, melt leaves the source with entrained crystals, after reaching
zircon and monazite saturation. A significant proportion of the rocks of the Peninsular Pluton have trace element concentrations
below those predicted by zircon and monazite saturation. In the case of the most leucocratic rocks all compositions are zircon
undersaturated; whilst the majority of the most mafic compositions are zircon oversaturated. However, in both cases, zircon
is commonly xenocrystic. Thus, the leucocratic rocks represent close to pure melts, which escaped their sources rapidly enough
that some very closely match the trace element disequilibrium melting model applied in this study. Zircon dissolution rates
allow the residency time for the melt in the source to be conservatively estimated at less than 500 years. 相似文献
5.
《Chemie der Erde / Geochemistry》2023,83(1):125924
Two-mica granite is the most common magmatic rock type in the Himalayan leucogranite belt, which has close relationship with rare metal mineralization. Its genesis is generally attributed to magmatic differentiation. In recent years, the mineral geochemical compositions are increasingly used to study magmatic differentiation, which are significant for deciphering the melt evolution and element migration processes. In this study, in-situ major and trace element and boron isotope compositions for tourmalines from two-mica granites in the Cuona and Cuonadong leucogranites in the Cuona area are conducted to determine microscopic changes in mineral assemblages and geochemical compositions. Analytical results show that the tourmalines in the Cuonadong leucogranite were crystallized earlier relative to the tourmalines in the Cuona leucogranite during magmatic differentiation. The volatile contents have a genetic relationship with incompatible elements in tourmaline, which is possibly responsible for the formation of tourmaline zonation and the enrichment of Sr, Zn, and Pb during magmatic differentiation. The B isotopic composition of tourmaline in the Cuona area suggests that the granitic magma was dominantly derived from the partial melting of the metasedimentary source rocks. Their B isotope variations likely resulted from fluid exsolution during B-rich melt evolution. High rare metal contents in tourmalines indicate that the two-mica granites in the Cuona area may have great mineralization potential. 相似文献
6.
Four muscovite-biotite granites from the Western Metamorphic Belt of South-eastern Australia have rare earth element patterns characterized by: (i) light rare earth element enrichment; (ii) slight Eu depletion; (iii) varying degrees of heavy rare earth element depletion. The rare earth element and major element chemistry of three of these muscovite-biotite granites (the Koetong, Lockharts and Yabba Granites) can be approximated very closely by a model involving 20% partial melting of an ultrametamorphosed pelitic sediment and contamination of this minimum melt by the residual material left after melting, in the ratio 60% melt: 40% residue. Granitoids can be very largely solid material at the time of emplacement.The other muscovite-biotite granite studied (the Hawksview Granite) has major and trace element characteristics which distinguish if from the other three granitoids and these differences are attributed to variations in source material at the site of melt generation.The rare earth element and major element chemistry of a garnet-cordierite gneiss from the Western Metamorphic Belt can be modelled assuming 5% partial melting of a pelitic metamorphic rock and contamination of the minimum melt by the residue in the ratio 30% melt: 70% residue.Separated granitic and biotitic portions of a migmatite from the Western Metamorphic Belt have rare earth element characteristics which are inconsistent with a simple partial-melting model, but it is suggested that re-equilibration following, or during, separation of the vein material could obscure the process by which the vein of the migmatite developed. It is however certain that the vein developed in situ from a pelitic meta-sediment leaving the biotite rich selvage, without the introduction of material from an external source.Leucogranites which crop out to the east of the Western Metamorphic Belt are high level intrusions of highly fractionated granitic melt. Their Sr isotopic characteristics and features of their major and trace element chemistry suggest that they derive from an igneous source and are not directly related to the granites and gneisses to the west. 相似文献
7.
福建沿海中生代变质带中花岗质岩石的地球化学 总被引：12，自引：1，他引：12
福建东南沿海中生代变质带的花岗质岩石分布于东山、晋江和莆田等广大地区。花岗岩中常包含黑云母、石榴子石或白云母。但地球化学研究表明，这些花岗岩属于钙碱性或高钾钙碱性，以低Rb、Zr、Hf、Nb、Y、Ga含量和Rb/Sr比值，以及高Ba、Sr丰度为特征，属于典型的Ⅰ型花岗岩。它们的稀土总量普遍较低，具有轻稀土富集、铕中等亏损的稀土分布模式。本带三个地区花岗岩的微量元素组成存在一定差异，但具有相似的Sr、Nd同位素组成，以高εNd(t)(-4.49～-3.15)和低ISr(0.7055-0.7074)、tDM(1.19-1.29Ga)为特征。地球化学研究显示本带花岗岩形成于相同的构造背景－大型边缘火山弧环境。其母岩浆是由类似于麻源群的古老火成变质岩部分熔融产生的熔融体与同期的幔源玄武质岩浆发生一定程度混合而成。不同地区或同一地区花岗岩地球化学组成上的差异是不同程度的部分熔融和结晶分异的结果。 相似文献
8.
Subduction-related Quaternary volcanic rocks from Solander and Little Solander Islands, south of mainland New Zealand, are porphyritic trachyandesites and andesites (58.20–62.19 wt% SiO2) with phenocrysts of amphibole, plagioclase and biotite. The Solander and Little Solander rocks are incompatible element enriched (e.g. Sr ~931–2,270 ppm, Ba ~619–798 ppm, Th ~8.7–21.4 ppm and La ~24.3–97.2 ppm) with MORB-like Sr and Nd isotopic signatures. Isotopically similar quench-textured enclaves reflect mixing with intermediate (basaltic-andesite) magmas. The Solander rocks have geochemical affinities with adakites (e.g. high Sr/Y and low Y), whose origin is often attributed to partial melting of subducted oceanic crust. Solander sits on isotopically distinct continental crust, thus excluding partial melting of the lower crust in the genesis of the magmas. Furthermore, the incompatible element enrichments of the Solander rocks are inconsistent with partial melting of newly underplated mafic lower crust; reproduction of their major element compositions would require unrealistically high degrees of partial melting. A similar argument precludes partial melting of the subducting oceanic crust and the inability to match the observed trace element patterns in the presence of residual garnet or plagioclase. Alternatively, an enriched end member of depleted MORB mantle source is inferred from Sr, Nd and Pb isotopic compositions, trace element enrichments and εHf ? 0 CHUR in detrital zircons, sourced from the volcanics. 10Be and Sr, Nd and Pb isotopic systematics are inconsistent with significant sediment involvement in the source region. The trace element enrichments and MORB-like Sr and Nd isotopic characteristics of the Solander rocks require a strong fractionation mechanism to impart the high incompatible element concentrations and subduction-related (e.g. high LILE/HFSE) geochemical signatures of the Solander magmas. Trace element modelling shows that this can be achieved by very low degrees of melting of a peridotitic source enriched by the addition of a slab-derived melt. Subsequent open-system fractionation, involving a key role for mafic magma recharge, resulted in the evolved andesitic adakites. 相似文献
9.
五垛山岩体是位于北秦岭地体东部的大型复式岩体，花岗岩类型复杂，并呈多期次侵位特征。对岩体中黑云母花岗岩和二长花岗岩的代表性样品进行锆石U-Pb 定年，显示其形成年龄为441~431Ma，部分样品保留古元古代至新元古代的继承锆石。五垛山岩体花岗岩具有高硅、低镁、富碱、弱过铝至强过铝特征。微量元素蛛网图显示富集Rb、Ba、K和Pb，亏损Nb、Ta、P、Ti。稀土元素的球粒陨石分配模式显示轻、重稀土分馏明显，δEu为0.36~1.45。五垛山岩体花岗岩的87Sr/86Sr(i)为0.70304~0.71290，εNd(t)值为-4.6~-1.9，两阶段Nd 模式年龄(TDM2)为1.34~1.58 Ga。稀土元素判别图显示这些花岗岩主要由部分熔融作用形成，同位素特征指示源区为北秦岭变沉积岩与幔源物质的混合，其中一部分花岗岩形成于下地壳的深度，另一部分花岗岩形成于中上地壳，而源区的幔源物质可能为早期侵位于地壳中的中基性岩体。结合北秦岭早古生代地壳演化背景，认为五垛山岩体花岗岩反映了早志留世北秦岭地体在后碰撞环境下加厚地壳的垮塌过程。 相似文献
10.
恰芒巴二云母花岗岩体位于特提斯喜马拉雅的西部, 岩石发育片麻状构造, 主要矿物组成为石英、钾长石、白云母和黑云母。LA-MC-ICP-MS U-Pb定年显示, 锆石年龄分布范围为35.1~17.3 Ma, 暗示较长时间的深熔作用过程, 其中最年轻的年龄(18.1±0.4 Ma)代表了花岗岩的最终结晶年龄。地球化学分析表明, 岩石具有高的SiO2(73.06%~73.79%)、Al2O3(14.73%~15.06%)和CaO(1.18%~1.24%)含量, 以及高的K2O/Na2O值(1.16~1.25)和A/CNK值(1.16~1.20), 属于高钾钙碱性过铝质花岗岩。岩石强烈富集Rb、Th、U和K, 而亏损Ba、Nb、Sr和Zr, 轻重稀土分馏较强(La/Yb)N=9.98~11.35, 并显示较弱的负Eu异常(δEu=0.70~0.74)。(87Sr/86Sr)i和εNd(t)值分别为0.742 298~0.743 092和-14.1~-14.0, 可与大喜马拉雅结晶杂岩(GHC)中变质沉积岩对比, 推测源岩为GHC变质沉积岩或与之成分相当的岩石。岩石(87Sr/86Sr)i值较低而Sr浓度较高, 随着Ba浓度的增加, Rb/Sr值基本不变, 与水致白云母部分熔融的趋势一致, 推测恰芒巴二云母花岗岩可能是水致白云母部分熔融的产物, 部分熔融作用可能与藏南拆离系的活动密切相关。 相似文献
11.
YANG Jiehu PENG Jiantang ZHAO Junhong FU Yazhou YANG Chen HONG Yinglong 《《地质学报》英文版》2012,86(1):131-152
Mesozoic granitic intrusions are widely distributed in the Nanling region,South China.Yanshanian granites are closely connected with the formation of tungsten deposits.The Xihuashan granite is a typica... 相似文献
12.
Whole rock elemental and Sr–Nd isotope geochemistry and in situ K-feldspar Pb isotope geochemistry were used to identify the sources involved in the genesis of Neoproterozoic granites from the Embu Terrane, Ribeira Belt, SE Brazil. Granite magmatism spanned over 200 Ma (810–580 Ma), and is dominated by crust-derived relatively low-T (850–750 °C, zircon saturation) biotite granites to biotite-muscovite granites. Two Cryogenian plutons show the least negative εNdt (−8 to −10) and highest mg# (30–40) of the whole set. Their compositions are strongly contrasted, implying distinct sources for the peraluminous (ASI ∼ 1.2) ∼660 Ma Serra do Quebra-Cangalha batholith (metasedimentary rocks from relatively young upper crust with high Rb/Sr and low Th/U) and the metaluminous (ASI = 0.96–1.00) ∼ 630 Ma Santa Catarina Granite. Although not typical, the geochemical signature of these granites may reflect a continental margin arc environment, and they could be products of a prolonged period of oceanic plate consumption started at ∼810 Ma. The predominant Ediacaran (595–580 Ma) plutons have a spread of compositions from biotite granites with SiO2 as low as ∼65% (e.g., Itapeti, Mauá, Sabaúna and Lagoinha granites) to fractionated muscovite granites (Mogi das Cruzes, Santa Branca and Guacuri granites; up to ∼75% SiO2). εNdT are characteristically negative (−12 to −18), with corresponding Nd TDM indicating sources with Paleoproterozoic mean crustal ages (2.0–2.5 Ga). The Guacuri and Santa Branca muscovite granites have the more negative εNdt, highest 87Sr/86Srt (0.714–0.717) and lowest 208Pb/206Pb and 207Pb/206Pb, consistent with an old metasedimentary source with low time-integrated Rb/Sr. However, a positive Nd–Sr isotope correlation is suggested by data from the other granites, and would be consistent with mixing between an older source predominant in the Mauá granite and a younger, high Rb/Sr source that is more abundant in the Lagoinha granite sample. The Ediacaran granites are coeval with profuse granite magmatism attributed to continental arc magmatism in northern Ribeira and Araçuaí belts. However, their evolved compositions with low mg# and dominantly peraluminous character are unlike those of magmatic arc granites, and they are more likely products of post-collisional magmatism or correspond to an inner belt of crust-derived granites. 相似文献
13.
Jean-Clair Duchesne Jean-Paul Liègeois Viorica Iancu Tudor Berza Dmitry I. Matukov Mihai Tatu Sergei A. Sergeev 《International Journal of Earth Sciences》2008,97(4):705-723
The Sichevita and Poniasca plutons belong to an alignment of granites cutting across the metamorphic basement of the Getic
Nappe in the South Carpathians. The present work provides SHRIMP age data for the zircon population from a Poniasca biotite
diorite and geochemical analyses (major and trace elements, Sr–Nd isotopes) of representative rock types from the two intrusions
grading from biotite diorite to biotite K-feldspar porphyritic monzogranite. U–Pb zircon data yielded 311 ± 2 Ma for the intrusion
of the biotite diorite. Granites are mostly high-K leucogranites, and biotite diorites are magnesian, and calcic to calc-alkaline.
Sr, and Nd isotope and trace element data (REE, Th, Ta, Cr, Ba and Rb) permit distinguishing five different groups of rocks
corresponding to several magma batches: the Poniasca biotite diorite (P1) shows a clear crustal character while the Poniasca granite (P2) is more juvenile. Conversely, Sichevita biotite diorite (S1), and a granite (S2*) are more juvenile than the other Sichevita granites (S2). Geochemical modelling of major elements and REE suggests that fractional crystallization can account for variations within
P1 and S1 groups. Dehydration melting of a number of protoliths may be the source of these magma batches. The Variscan basement, a
subduction accretion wedge, could correspond to such a heterogeneous source. The intrusion of the Sichevita–Poniasca plutons
took place in the final stages of the Variscan orogeny, as is the case for a series of European granites around 310 Ma ago,
especially in Bulgaria and in Iberia, no Alleghenian granitoids (late Carboniferous—early Permian times) being known in the
Getic nappe. The geodynamical environment of Sichevita–Poniasca was typically post-collisional of the Variscan orogenic phase. 相似文献
14.
《International Geology Review》2012,54(10):1121-1149
Peraluminous intrusives of the Akum-Bamenda Massif, Pan-African Belt, Central Cameroon, were synkinematically emplaced in a Pan-African sinistral strike-slip shear zone. The rock sequences consist of medium-grained leucogranites, fine-grained leucogranites, and orthogneisses of biotite granite composition; in aggregate, they cover a range from about 65 to 74 wt.% SiO2, defining a continuous chemical evolutionary trend and displaying characteristics of the high-K and medium calc-alkaline series. Leucogranites are strongly peraluminous (A/CNK > 1.1) and plot in the field of S-type granites, whereas orthogneisses are metaluminous and plot in the field of I-type granitoids. Major and trace element compositions and the Rb/Sr isotopes of the leucogranites indicate crustal derivation by remelting of a composite metapelite?+?metagreywacke protolith similar to the metasedimentary rocks of the central domain of the Cameroon Pan-African North-Equatorial fold belt. 相似文献
15.
大兴安岭中段五岔沟地区蛤蟆沟林场A型花岗岩年代学、岩石地球化学及构造背景研究 总被引：5，自引：0，他引：5
对大兴安岭中部五岔沟镇蛤蟆沟林场花岗岩体研究结果表明,蛤蟆沟林场花岗岩体主要岩性组合为碱长花岗岩、黑云母正长花岗岩、黑云母二长花岗岩和花岗斑岩,其LA-ICP-MS锆石U-Pb测年结果为136.97±0.99Ma、135.85±0.78Ma、126.04±0.48Ma和135.87±0.49Ma,表明其形成时代为早白垩世。岩石主量元素具有富硅、富钾的特征,A/CNK值介于0.997~1.05之间,稀土元素相对富集轻稀土元素、亏损重稀土元素,并具有较为明显的Eu负异常,微量元素相对富集大离子亲石元素和高场强元素,贫Ba、Sr、Ti、P,这些特征表明蛤蟆沟林场花岗岩为铝质A型花岗岩,是中下地壳岩石部分熔融的产物,形成于大陆裂谷或板内构造环境（A1型花岗岩）,代表了伸展的大地构造背景,可能是受到古太平洋俯冲和蒙古鄂霍次克洋闭合的双重影响。 相似文献
16.
REE-Depleted Leucogranites, Black Hills, South Dakota: a Consequence of Disequilibrium Melting of Monazite-Bearing Schists 总被引：4，自引：1，他引：3
Two isotopically distinct but otherwise chemically similar leucogranitesuites in the Proterozoic Horney Peak Granite, Black Hills,South Dakota, have contrasting light rare earth element (LREE)concentrations. Most samples of a relatively 18O-depleted suitehave LREE- enriched, chondrite-normalized patterns, typicalof melts derived from metasedimentary protoliths, whereas allsamples of the regionally significant, relatively 18O-enrichedsuite have LREE-depleted patterns. The latter patterns are interpretedto have resulted from disequilibrium melting of schists. Monaziteand perhaps other accessory minerals remained armored by biotiteand garnet which did not partake in the muscovite dehydration-meltingreaction that produced LREE-depleted melts. The REE concentrationsin the LREE-depleted samples are below saturation levels formonazite at reasonable melting temperatures and melt water contents,whereas the REE concentrations in the LREE-enriched samplesyield 700–800C monazite saturation temperatures, reasonablefor biotite dehydration-melting reactions. LREE depletions,analogous to those in the LREE-depleted granites, are also foundin leucosomes of partially molten schists, thought to be theprotolith for the granite. In contrast, the melanosomes holdthe accessory minerals and bulk of the LREEs. KEY WORDS: accessory minerals; leucogranites; Black Hills; monazite; partial melting
*Corresponding author at Department of Geological Sciences, University of Missouri. Telephone: 314-884-6463. Fax: 314-882-5458. e-mail: geolpin{at}showme.missouri.edu. 相似文献
17.
High- and Low-Temperature I-type Granites 总被引：4，自引：0，他引：4
B. W. CHAPPELL C. J. BRYANT D. WYBORN A. J. R. WHITE I. S. WILLIAMS 《Resource Geology》1998,48(4):225-235
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
18.
喜马拉雅碰撞造山带新生代地壳深熔作用与淡色花岗岩 总被引：12，自引：10，他引：2
自从印度-欧亚大陆碰撞以来,伴随着构造演化和温度-压力-成分(P-T-X)的变化,喜马拉雅造山带中下地壳变质岩发生不同类型的部分熔融反应,形成性质各异的过铝质花岗岩。这些花岗岩在形成时代、矿物组成、全岩元素和放射性同位素地球化学特征上都表现出巨大的差异性。始新世构造岩浆作用形成高Sr/Y二云母花岗岩和演化程度较高的淡色花岗岩和淡色花岗玢岩,它们具有相似的Sr-Nd同位素组成,是碰撞早期增厚下地壳部分熔融的产物。渐新世淡色花岗岩主要为演化程度较高的淡色花岗岩,可能指示了喜马拉雅造山带的快速剥露作用起始于渐新世。早中新世以来的淡色花岗岩是喜马拉雅造山带淡色花岗岩的主体,是变泥质岩部分熔融的产物,包含两类部分熔融作用——水致白云母部分熔融作用(A类)和白云母脱水熔融作用(B类)。这两类部分熔融作用形成的花岗质熔体在元素和同位素地球化学特征上都表现出明显的差异性,主要受控于两类部分熔融作用过程中主要造岩矿物和副矿物的溶解行为。这些不同期次的地壳深熔作用都伴随着高分异淡色花岗岩,伴随着关键金属元素(Nb、Ta、Sn、Be等)的富集,是未来矿产勘探的重要靶区。新的观测结果表明:在碰撞造山带中,花岗岩岩石学和地球化学性质的变化是深部地壳物质对构造过程响应的结果,是深入理解碰撞造山带深部地壳物理和化学行为的重要岩石探针。 相似文献
19.
Satoshi Saito Makoto Arima Takashi Nakajima Kenichiro Tani Takashi Miyazaki Ryoko Senda Qing Chang Toshiro Takahashi Yuka Hirahara Jun-Ichi Kimura 《Contributions to Mineralogy and Petrology》2012,163(4):611-629
The Miocene Kaikomagatake pluton is one of the Neogene granitoid plutons exposed in the Izu Collision Zone, which is where
the juvenile Izu-Bonin oceanic arc is colliding against the mature Honshu arc. The pluton intrudes into the Cretaceous to
Paleogene Shimanto accretionary complex of the Honshu arc along the Itoigawa-Shizuoka Tectonic Line, which is the collisional
boundary between the two arcs. The pluton consists of hornblende–biotite granodiorite and biotite monzogranite, and has SiO2 contents of 68–75 wt%. It has high-K series compositions, and its incompatible element abundances are comparable to the average
upper continental crust. Major and trace element compositions of the pluton show well-defined chemical trends. The trends
can be interpreted with a crystal fractionation model involving the removal of plagioclase, biotite, hornblende, quartz, apatite,
and zircon from a potential parent magma with a composition of ~68 wt% SiO2. The Sr isotopic compositions, together with the partial melting modeling results, suggest that the parent magma is derived
by ~53% melting of a hybrid lower crustal source comprising ~30% Shimanto metasedimentary rocks of the Honshu arc and ~70%
K-enriched basaltic rocks of the Izu-Bonin rear-arc region. Together with previous studies on the Izu Collision Zone granitoid
plutons, the results of this study suggest that the chemical diversity within the parental magmas of the granitoid plutons
reflects the chemical variation of basaltic sources (i.e., across-arc chemical variation in the Izu-Bonin arc), as well as
a variable contribution of the metasedimentary component in the lower crustal source regions. In addition, the petrogenetic
models of the Izu Collision Zone granitoid plutons collectively suggest that the contribution of the metasedimentary component
is required to produce granitoid magma with compositions comparable to the average upper continental crust. The Izu Collision
Zone plutons provide an exceptional example of the transformation of a juvenile oceanic arc into mature continental crust. 相似文献
20.
五垛山岩体是位于北秦岭地体东部的大型复式岩体,花岗岩类型复杂,并呈多期次侵位特征。对岩体中黑云母花岗岩和二长花岗岩的代表性样品进行锆石U-Pb 定年,显示其形成年龄为441~431Ma,部分样品保留古元古代至新元古代的继承锆石。五垛山岩体花岗岩具有高硅、低镁、富碱、弱过铝至强过铝特征。微量元素蛛网图显示富集Rb、Ba、K和Pb,亏损Nb、Ta、P、Ti。稀土元素的球粒陨石分配模式显示轻、重稀土分馏明显,δEu为0.36~1.45。五垛山岩体花岗岩的87Sr/86Sr(i)为0.70304~0.71290,εNd(t)值为-4.6~-1.9,两阶段Nd 模式年龄(TDM2)为1.34~1.58 Ga。稀土元素判别图显示这些花岗岩主要由部分熔融作用形成,同位素特征指示源区为北秦岭变沉积岩与幔源物质的混合,其中一部分花岗岩形成于下地壳的深度,另一部分花岗岩形成于中上地壳,而源区的幔源物质可能为早期侵位于地壳中的中基性岩体。结合北秦岭早古生代地壳演化背景,认为五垛山岩体花岗岩反映了早志留世北秦岭地体在后碰撞环境下加厚地壳的垮塌过程。 相似文献