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1.
At Telões, a subaluminous medium- to coarse-grained porphyritic biotite granite, crops out along the Vila Real NNE–SSW fault. It is a post-tectonic granite of 299±3 Ma old given by U–Pb isotopic data on zircon. It contains metaluminous to subaluminous tonalitic, granodioritic and monzogranitic enclaves. All granitoids have Fe 2+-biotite and some enclaves contain magnesiohornblende and subsolidus actinolite. Monzogranitic enclaves show obvious similarities to the host granite. Linear array between enclaves and host granite is observed in Rb–Sr typical isochron diagram and gives the age of 286±11 Ma and ( 87Sr/ 86Sr) 0=0.7063±0.0011. Microgranular enclaves have δ18O values similar to those of the hosting granite. Microgranular enclaves are hybrid rocks probably formed by mixing between a tonalitic enclave magma and a host granite magma as supported by the modelling of major and trace elements. The similar isotopic signatures suggest a subsequent partial equilibration of the enclaves and granite magmas. 相似文献
2.
The Rooiberg Group is a 6-km-thick sequence of mostly volcanic rocks, which represent the first phase of magmatic activity associated with the Bushveld Complex. These strata include, in ascending stratigraphic order, the Dullstroom, Damwal, Kwaggasnek, and Schrikkloof Formations. Units of the lower Dullstroom Formation range from basalts to andesites and comprise two compositional suites: high Ti and low Ti. Compositional data indicate that melts represented by the overlying, more siliceous volcanic rocks, which include dacites and rhyolites, were derived from low Ti melts by fractional crystallization and assimilation of crustal material (AFC processes). Rb–Sr isotopic data (28 samples) for units of the Dullstroom and Damwal Formations loosely constrain a crystallization age of 2071+94/−65 Ma (these errors and those below: 95% confidence limits), which agrees with previously reported age data. These data suggest an initial value for 87Sr/86Sr of 0.70655+0.00087/−0.00051 for the Rooiberg Group. In contrast, Rb–Sr isotopic compositions of six samples of the Kwaggasnek Formation indicate post-crystallization alteration, which was probably associated with the Lebowa Granite Suite of the Bushveld Complex. Sm–Nd isotopic data (29 samples) for volcanic units of the Rooiberg Group provide a poorly constrained age of 1837+360/−320 Ma with an initial value for 143Nd/144Nd of 0.50976+0.00026/−0.00035. These Rb–Sr and Sm–Nd isotopic compositions are similar to those indicated for melts that crystallized to form the Rustenburg Layered Suite (RLS) of the Bushveld Complex. Extruded and intruded melts probably were derived from the same or similar sources and may have resided in the same magma chambers before emplacement. 相似文献
3.
In order to provide mantle and crustal constraints during the evolution of the Colombian Andes, Sr and Nd isotopic studies were performed in xenoliths from the Mercaderes region, Northern Volcanic Zone, Colombia. Xenoliths are found in the Granatifera Tuff, a deposit of Cenozoic age, in which mantle- and crustal-derived xenoliths are present in bombs and fragments of andesites and lamprophyres compositions. Garnet-bearing xenoliths are the most abundant mantle-derived rocks, but websterites (garnet-free xenoliths) and spinel-bearing peridotites are also present in minor amounts. Amphibolites, pyroxenites, granulites, and gneisses represent the lower crustal xenolith assemblage. Isotopic signatures for the mantle xenoliths, together with field, petrographic, mineral, and whole-rock chemistry and pressure–temperature estimates, suggest three main sources for these mantle xenoliths: garnet-free websterite xenoliths derived from a source region with low P and T (16 kbar, 1065 °C) and MORB isotopic signature, 87Sr/ 86Sr ratio of 0.7030, and 143Nd/ 144Nd ratio of 0.5129. Garnet-bearing peridotite and websterite xenoliths derived from two different sources in the mantle: i) a source with intermediate P and T (29–35 kbar, 1250–1295 °C) conditions, similar to that of sub-oceanic geotherm, with an OIB isotopic signature ( 87Sr/ 86Sr ratio of 0.7043 and 143Nd/ 144Nd ratio of 0.5129); and ii) another source with P and T conditions similar to those of a sub-continental geotherm (>38 kbar, 1140–1175 °C) and OIB isotopic characteristics ( 87Sr/ 86Sr ratio=0.7041 and 143Nd/ 144Nd ratio=0.5135). 相似文献
4.
Cerro Redondo is an ancient cinder cone now almost completely eroded, sited over a sill that corresponds to a sub-volcanic magma chamber, in Santa Cruz province, Patagonia, Argentina. It is composed of Pliocene-Pleistocene alkaline basalt containing spinel-facies lherzolite and harzburgite mantle xenoliths. Core compositions of pyroxenes indicate temperatures of 823 °C to 1043 °C and pressures of 12.4 kb to 21.4 kb. Based on P– T estimates, petrographic, geochemical, and isotopic characteristics, we propose that Cerro Redondo xenoliths come from a thick homogeneous mantle column (36 km to 63 km depth), and present different degrees of basalt infiltration. A simple mixing model based on Sr isotopes was used to quantify the host basalt infiltration, and contamination values of 0.0%, 0.2%, 3%, and 12% were obtained for samples X-F, X-D, X-C, and X-B, respectively. For unknown reasons, samples X-G and X-E suffered selective isotopic and trace element modification, respectively, associated with 1% of basalt infiltration. Sample X-F best represents the sub-continental lithospheric mantle column, conserving primary equilibrium textures with sharp grain boundaries, and having TiO 2, CaO, Na 2O, K 2O, and P 2O 5 contents lower than average spinel lherzolite, flat chondrite-normalized REE pattern, and 87Sr/ 86Sr and 143Nd/ 144Nd ratios of 0.70519 and 0.51297, respectively. This sample records a decoupling of the Sr–Nd system where Sr ratios increase at constant Nd ratios, possibly caused by chromatographic processes. Its 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb ratios are 17.987, 15.556, and 37.959, respectively. As the interaction with the host basalt increases, xenoliths show a gradual increase of disequilibrium textures such as reaction rims and exsolution lamellae in orthopyroxene and clinopyroxene, and increase of TiO 2, CaO, Al 2O 3, Na 2O, K 2O, P 2O 5, LREE, and incompatible element concentrations. The Sr–Nd system shows an unusual positive trend from the unmodified sample X-F toward the host basalt isotope composition with 87Sr/ 86Sr and 143Nd/ 144Nd ratios of 0.70447 and 0.51279, respectively, while 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb ratios tend to increase toward those of the host basalt (18.424, 15.648, and 38.728, respectively) as the xenolith–basalt interaction increases. The basalt–xenolith reaction probably started during the transport of the xenoliths to the surface, and continued during the residence of xenoliths in the sub-volcanic magma chamber of Cerro Redondo. 相似文献
5.
Deformed Hercynian peraluminous granitoids ranging from tonalite to granite crop out in the Rebordelo–Agrochão area, northern Portugal and some of them contain tonalitic and granodioritic enclaves. Variation diagrams of major and trace elements of the rocks, biotites and sphenes show fractionation trends. The most- and the least-deformed samples of granite and their biotites also define fractionation trends. There is decrease in all rare earth element (REE) contents and increase in the Eu anomaly in REE patterns from the most- to the least-deformed samples of granite. All the granitoids define a whole-rock Rb–Sr errorchron. A whole-rock Rb–Sr isochron for the least-deformed samples of granite yields an age of 357±9 Ma and an initial 87Sr/ 86Sr ratio of 0.7087±0.0007. Geochemical modelling suggests that the tonalitic magma evolved by AFC (fractional crystallization of magnesiohornblende, plagioclase, quartz, biotite and ilmenite, and assimilation of metasediments) to originate tonalitic and granodioritic enclaves, granodiorite and granite. δ18O values support this mechanism. The tonalite is hybrid and derived by interaction of a mantle-derived magma and crustal materials. 相似文献
6.
The magmatism at the axial zone of the middle Okinawa Trough, a young continental back-arc basin, comprises a bimodal basaltic–rhyolitic suite, accompanied by minor intermediate rocks. We report major and trace element and Sr–Nd isotopic data for the intermediate to silicic suites, to provide constraints on their petrogenesis. The rhyolites, recovered as lava and pumice, fall into three geochemical groups (type 1, 2, and 3 rhyolites). Type 1 rhyolites have 87Sr/ 86Sr (0.7040–0.7042) and 143Nd/ 144Nd (0.5128–0.5129) identical to those of associated basalts, and are characterized by highly fractionated REE patterns. Petrogenesis of type 1 rhyolites is explicable in terms of fractional crystallization of the associated basalt. In contrast, type 2 rhyolites and andesite have slightly higher 87Sr/ 86Sr (0.7044–0.7047) but similar 143Nd/ 144Nd (0.5128) compared to those of the basalts. The compositions of type 2 rhyolite and andesite can be explained by assimilation and fractional crystallization (AFC) processes of the basalt magma; quantitative analysis suggests assimilation/fractional crystallization ( Ma/ Mc) ratios of ≤0.05. Hybrid andesite generated by mixing of evolved basalt and type 1 rhyolite is also present. We emphasize that mechanical extension in this part of the Okinawa Trough involves gabbroic lower crust that resulted from fractionation of mantle-derived basaltic magmas. Type 3 rhyolite occurs only as pumice, which makes its derivation questionable. This rhyolite has major and trace element compositions and Sr–Nd isotopic ratios, which suggests that it may be derived from volcanic activity on the southern Ryukyu volcanic front, and arrived in the Okinawa Trough by drifting on the Kuroshio Current. 相似文献
7.
The Storgangen orebody is a concordantly layered, sill-like body of ilmenite-rich norite, intruding anorthosites of the Rogaland Intrusive Complex (RIC), SW Norway. 17 zircon grains were separated from ca. 5 kg of sand-size flotation waste collected from the on-site repository from ilmenite mining. These zircons were analysed for major and trace elements by electron microprobe, and for U–Pb and Lu–Hf isotopes by laser ablation microprobe plasma source mass spectrometry. Eight of the zircons define a well-constrained (MSWD=0.37) concordant population with an age of 949±7 Ma, which is significantly older than the 920–930 Ma ages previously reported for zircon inclusions in orthopyroxene megacrysts from the RIC. The remaining zircons, interpreted as inherited grains, show a range of 207Pb/ 206Pb ages up to 1407±14 Ma, with an upper intercept age at ca. 1520 Ma. The concordant zircons have similar trace element patterns, and a mean initial Hf isotope composition of 176Hf/ 177Hf 949 Ma=0.28223±5 ( Hf=+2±2). This is similar to the Hf-isotope composition of zircons in a range of post-tectonic Sveconorwegian granites from South Norway, and slightly more radiogenic than expected for mid-Proterozoic juvenile crust. The older, inherited zircons show Lu–Hf crustal residence ages in the range 1.85–2.04 Ga. One (undated) zircon plots well within the field of Hf isotope evolution of Paleoproterozoic rocks of the Baltic Shield. These findings indicate the presence of Paleoproterozoic components in the deep crust of the Rogaland area, but do not demonstrate that such rocks, or a Sveconorwegian mantle-derived component, contributed significantly to the petrogenesis of the RIC. If the parent magma was derived from a homogeneous, lower crustal mafic granulite source, the lower crustal protolith must be at least 1.5 Ga old, and it must have an elevated Rb/Sr ratio. This component would be indistinguishable in Sr, Nd and Hf isotopes from some intermediate mixtures between Sveconorwegian mantle and Paleoprotoerzoic felsic crust, but it cannot account for the initial 143Nd/ 144Nd of the most primitive, late Sveconorwegian granite in the region, without the addition of mantle-derived material. 相似文献
8.
This study presents new major and trace element and Sr–Nd isotopic results for a suite of Miocene–Recent mafic lavas from the Kivu volcanic province in the western branch of the East African Rift. These lavas exhibit a very wide range in chemical and isotopic characteristics, due to a lithospheric mantle source region that is heterogeneous on a small scale, probably <1 km. The chemical and isotopic variations are mostly geographically controlled: lavas from Tshibinda volcano, which lies on a rift border fault on the northwestern margin of the province, have higher values of 87Sr/ 86Sr, (La/Sm) n, Ba/Nb, and Zr/Hf than the majority of Kivu (Bukavu) samples. The range of 87Sr/ 86Sr at Tshibinda (0.70511–0.70514) overlaps some compositions found in the neighboring Virunga province, while Bukavu group lavas include the lowest 87Sr/ 86Sr (0.70314) and highest Nd (+7.6) yet measured in western rift lavas. The Tshibinda compositions trend towards a convergence for Sr–Nd–Pb isotopic values among western rift lavas. Among Kivu lavas, variations in 143Nd/ 144Nd correlate with those for certain incompatible trace element ratios (e.g., Th/Nb, Zr/Hf, La/Nb, Ba/Rb), with Tshibinda samples defining one compositional extreme. There are covariations of isotopic and trace element ratios in mafic lavas of the East African Rift system that vary systematically with geographic location. The lavas represent a magmatic sampling of variations in the underlying continental lithospheric mantle, and it appears that a common lithospheric mantle (CLM) source is present beneath much of the East African Rift system. This source contains minor amphibole and phlogopite, probably due to widespread metasomatic events between 500 and 1000 Ma. Lava suites which do not show a strong component of the CLM source, and for which the chemical constraints also suggest the shallowest magma formation depths, are the Bukavu group lavas from Kivu and basanites from Huri Hills, Kenya. The inferred extent of lithospheric erosion therefore appears to be significant only beneath these two areas, which is generally consistent with lithospheric thickness variations estimated from gravity and seismic studies. 相似文献
9.
Volumetrically minor microsyenites, alkali microgranite and related trachytic dykes intrude early Pliocene OIB-like alkali basaltic and basanitic flows of the Meseta del Lago Buenos Aires in Central Patagonia (47°S–71°30′W), and occur together with scarce trachytic lava flows. Whole-rock K–Ar ages between 3.98 and 3.08 Ma indicate that the emplacement of these felsic rocks occurred more or less synchronously with that of the post-plateau basaltic sequence that they intrude, during a bimodal mafic–felsic magmatic episode devoid of intermediate compositions. Chemically, these rocks have A 1-type granitoid affinities and are characterized by high silica and alkali contents (60–68 wt.% SiO 2; 8.7–10.8 wt.% Na 2O + K 2O), major and trace elements patterns evidencing evolution by low-pressure fractional crystallization, and Sr and Nd isotopic signatures similar to those of coeval basalts (( 87Sr/ 86Sr) o = 0.70488–0.70571; ( 143Nd/ 144Nd) o = 0.512603–0.512645). Nevertheless, some of them have the most radiogenic Sr values ever reported for a magmatic rock in the Meseta and even in the whole Neogene Patagonian Plateau Lavas province (( 87Sr/ 86Sr) o = 0.70556–0.70571; ( 143Nd/ 144Nd) o = 0.512603–0.512608). In addition, very high contents of strongly incompatible elements in the most evolved rocks, together with Sr isotopic ratios higher than those of coeval basalts, suggest the occurrence of open-system magmatic processes. Continuous fractional crystallization from a primitive basaltic source, similar to post-plateau coeval basalts, towards alkali granites combined with small rates of assimilation of host Jurassic tuffs (AFC) in a shallow magmatic reservoir, best explains the geochemical and petrographic features of the felsic rocks. Therefore, A 1-type magmatic rocks can be generated by open-system crystallization of deep asthenospheric melts in back-arc tectonic settings. In Central Patagonia, these 3–4 Ma old alkaline intrusions occur aligned along a N160–170 trending lineament, the Zeballos Fault Zone, stacking the morphotectonic front of one segment of the Patagonian Cordillera. Intrusion along this fault zone occurred during the onset of a new transtensional or extensional event in the area, related to major regional tectonics occurring in possible relation with the collision of one segment of the Chile Spreading Ridge with the trench. 相似文献
10.
沙德盖岩体位于华北克拉通北缘中段、哈达门沟大型金（钼）矿田范围内。首次利用锆石 SHRIMP U-Pb法对其定年,获得15个锆石颗粒 206Pb/ 238U年龄的加权平均值（221.6±2.1） Ma（MSWD=1.6）,表明岩体侵位于印支中期。岩石地球化学特征表现为高硅（SiO 2质量分数为71.21%～73.67%）、富钾（K 2O/Na 2O为1.01～1.37）、富碱（K 2O+Na 2O为8.23%～9.96%）、弱过铝质（Al 2O 3为13.11%～14.31%）,里特曼指数 σ=2.43～3.52,钙碱性-碱性;富集轻稀土（LREE/HREE为17.68～14.92,（La/Yb）| N为22.85～16.58）和Eu略亏损（ δEu=0.95～0.93）;微量元素亏损Nb、Ta、P、Ti、Sr,富集K、La、Ce、Hf等元素,具有A型花岗岩特征;( 87Sr/ 86Sr)i=0.705 31～0.702 29,( 143Nd/ 144Nd) i=0.511 682～0.511 620, εNd( t)=-13.1～-14.3,具有壳源特点;钕两阶段模式年龄 T2DM=2 061～2 160 Ma,在铅构造模式图上样品投点于地幔与下地壳之间。综合分析认为其形成于同碰撞向后碰撞构造体制转换,伸展构造和幔源基性岩浆的底侵导致早期古老基底地壳部分熔融,很可能是形成沙德盖岩体的主要动力机制。华北克拉通北缘印支期构造岩浆活动及成矿作用是普遍存在的。 相似文献
11.
The Columbia seamount 825 km offshore from Brazil at 20°S lies on the east–west ‘trace’ of the Trindade hotspot. Continental and oceanic magmatism believed to have originated with this hotspot is alkalic and SiO 2-undersaturated, and dates from 85 Ma in southern Brazil to <3 Ma on the islands of Trindade and Martin Vaz 1100 km offshore. An ankaramite (clinopyroxene 16 vol%) dredged from Columbia seamount (est. 10 Ma) conforms to this geochemistry with SiO 2-undersaturated Al-rich clinopyroxene (8–13 wt.% Al 2O 3) and rhönite. Clinopyroxene isotopic compositions are 87Sr/ 86Sr=0.703900, 143Nd/ 144Nd=0.512786, 206Pb/ 204Pb=19.190, 207Pb/ 204Pb=15.045, and 208Pb/ 204Pb=39.242 — resembling those for Trindade, except for slightly higher 207Pb/ 204Pb. The isotopic composition and abundance ratios among weathering-resistant Nb, La, and Yb suggest that Columbia seamount magmatism represents the present-day Trindade plume, but 10 million years earlier and perhaps when the plume manifested a signature of ‘contamination’ from subducted sediments. The Columbia seamount analyses provide the first quantitative assessment for the Trindade hotspot trace existing between the Brazil margin and Trindade, strengthening the case for a continuum of magmatism extending from the 85 Ma Brazilian igneous provinces of Poxoréu and Alto Paranaiba. 相似文献
12.
The Rio Espinharas pluton, northeastern Brazil, belongs to the shoshonitic series and consists mainly of syenogranite, quartz–monzonite and porphyritic quartz–monzonite, but diorite, quartz–monzodiorite, quartz–syenite and microsyenogranite also occur containing microgranular enclaves, except for the diorite. Most variation diagrams of rocks, amphiboles, biotites and allanites show linear trends, but K, Zr, Sr and Ba of rocks display curved scattered trends. The rocks ranging from diorite to syenogranite define a pseudo-errorchron and have similar REE patterns. Syenogranite and microsyenogranite are derived from two distinct pulses of granite magma with initial 87Sr/ 86Sr ratio of 0.7083±0.0003 and 0.7104±0.0007, respectively. Modelling of major and trace elements shows that the syenogranite evolved by fractional crystallization of plagioclase, microcline, edenite, biotite and titanite, whereas quartz–monzonite, porphyritic quartz–monzonite, quartz–monzodiorite and quartz–syenite resulted from simple mixing between an upper mantle-derived dioritic magma and the upper crust-derived syenogranite magma. Dioritic enclaves are globules of a mafic magma from the upper mantle. 相似文献
13.
Lamprophyres consisting mainly of diopside, phlogopite and K-feldspar formed in the early Tertiary around 60 Ma in the Beiya area and are characterized by low SiO 2 ± 46–50 wt.%), Rb (31–45 ppm) and Sr (225–262 ppm), high Al 2O 3, (11.2–13.1 wt.%), CaO (8.0–8.7 wt.%), MgO (11.5–12.1 wt.%), K 2O(4.9–5.5 wt.%), TiO 2 (2.9–3.3 wt.%) and REE (174–177 ppm), and compatible elements (e.g. Sc, Cr and Ni) and HSF elements (e.g. Th, U, Zr, Nb, Ta, Ti and Y), and low 143Nd/ 144Nd 0.512372–0.512536, middle 87Sr/ 86Sr 0.707322–0.707395, middle 206Pb/ 204Pb 18.50–18.59, 207Pb/ 204Pb 15.60–15.65 and 208Pb/ 204Pb 38.75–38.8. These rocks developed peculiar quartz megacrysts with poly-layer reaction zones, melt inclusions, and partial melted K-feldspar and plagioclase inclusions, and plastic shapes. Important features of these rocks include: (1) hybrid composition of elements, (2) abrupt increase of SiO 2 content of the melt, recorded by zoned diopside, (3) development of sanidine and aegirine-augite reaction zones, (4) alkaline melt and partial melted K-feldspar and plagioclase inclusions, (5) deformed quartz inclusions associated with quartz megacrysts, (6) the presence of quartz megacrysts in plastic shape with their parent melts, (7) the occurrence of olivine, high-MgO ilmenite and spinel inclusions within earlier formed diopside, phlogopite and magnetite. Median 87Sr/ 86Sr values between Tertiary alkaline porphyries in the Beiya area and the western Yunnan and Tertiary basalt in the western Yunnan indicate that the Beiya lamprophyre melts were derivative and resulted from the mixing between basic melts that were related to the partial melting of phenocrysts of spinel iherzolite from a mantle source. The alkaline melts originated from partial melting along the Jinshajiang subduction ductile shear zone at the contact between the buried Palaeo-Tethyan oceanic lithosphere and the upper mantle lithosphere. The alkaline melts are composed of 65% sanidine (Or 70Ab 28An 2) and 35% SiO 2. The melt mixing occurred in magma chambers in the middle-shallow crust at 8–10 km before the derivative lamprophyre melts intruded into the shallow cover in Beiya area. This mixing of basic and alkaline melts might represent a general process for the formation of lamprophyre in the western Yunnan. 相似文献
14.
Sr–Nd–Pb isotope ratios of alkaline mafic intra-plate magmatism constrain the isotopic compositions of the lithospheric mantle along what is now the eastern foreland or back arc of the Cenozoic Central Andes (17–34°S). Most small-volume basanite volcanic rocks and alkaline intrusive rocks of Cretaceous (and rare Miocene) age were derived from a depleted lithospheric mantle source with rather uniform initial 143Nd/ 144Nd ( 0.5127–0.5128) and 87Sr/ 86Sr ( 0.7032–0.7040). The initial 206Pb/ 204Pb ratios are variable (18.5–19.7) at uniform 207Pb/ 204Pb ratios (15.60 ± 0.05). A variety of the Cretaceous depleted mantle source of the magmatic rocks shows elevated Sr isotope ratios up to 0.707 at constant high Nd isotope ratios. The variable Sr and Pb isotope ratios are probably due to radiogenic growth in a metasomatized lithospheric mantle, which represents the former sub-arc mantle beneath the early Palaeozoic active continental margin. Sr–Nd–Pb isotope signatures of a second mantle type reflected in the composition of Cretaceous (one late Palaeozoic age) intra-plate magmatic rocks ( 143Nd/ 144Nd 0.5123, 87Sr/ 86Sr 0.704, 206Pb/ 204Pb 17.5–18.5, and 207Pb/ 204Pb 15.45–15.50) are similar to the isotopic composition of old sub-continental lithospheric mantle of the Brazilian Shield. Published Nd and Sr isotopic compositions of Mesozoic to Cenozoic arc-related magmatic rocks (18–40°S) represent the composition of the convective sub-arc mantle in the Central Andes and are similar to those of the Cretaceous (and rare Miocene) intra-plate magmatic rocks. The dominant convective and lithospheric mantle type beneath this old continental margin is depleted mantle, which is compositionally different from average MORB-type depleted mantle. The old sub-continental lithospheric mantle did not contribute to Mesozoic to Cenozoic arc magmatism. 相似文献
15.
大兴安岭北段新林地区晚古生代花岗岩主要出露在大乌苏和富西里附近,岩性主要为二长花岗岩,另有少量花岗闪长岩。对其中二长花岗岩样品进行LA-ICP-MS锆石U-Pb测年表明,大乌苏和富西里岩体侵位年龄分别为（303.7±2.2）和（300.5±0.5）Ma,均为晚石炭世岩浆活动的产物。花岗岩具有富硅（ w（SiO 2）为66.77%~75.85%）、富碱（ w（Na 2O+K 2O）为7.41%~8.69%）、高铝（ w（Al 2O 3）为12.90%~16.22%）,低MgO、CaO、TiO 2的特点,属于钙碱性系列;铝饱和指数（ A/ CNK）为1.06~1.44,为过铝质岩石;镜下未见原生白云母、堇青石、石榴石等富铝矿物,不同于富铝的S型花岗岩;而 w（P 2O 5）与 w（SiO 2）负相关,呈I型花岗岩特征;富集LREE和Ba、Rb、K等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,与后造山I型花岗岩特征相似,应形成于拉张的构造环境。花岗岩的 87Sr/ 86Sr为0.712 938、 143Nd/ 144Nd为0.512 386,（ 87Sr/ 86Sr） i值为0.704 4, εNd（ t）值为-1.09, TDM2=1 172 Ma,源区物质主要为中-新元古代从亏损地幔增生的地壳物质。结合区域研究成果,大兴安岭新林地区晚石炭世岩浆侵位活动与额尔古纳-兴安地块和松嫩地块碰撞拼合后岩石圈伸展环境有关。 相似文献
16.
Whole-rock Nd and Sr isotopic compositions of the mafic-ultramafic complex near Finero demonstrate that the magma was derived from a depleted, perhaps MORB-type mantle reservoir. The Sm-Nd data for the Amphibole Peridotite unit can be interpreted as an isochron with an apparent age of 533 ± 20 Ma, which is consistent with a 207Pb/ 206Pb evaporation age of 549 ± 12 Ma of a single zircon grain from the Internal Gabbro unit. However, the interpretation of these apparent ages remains open to question. We therefore retain the alternative hypotheses that the intrusion occurred either about 533 or 270 Ma ago, the latter being the most likely age of emplacement of the much larger magma body near Balmuccia (Val Sesia). The implication of the older emplacement age (if correct) would be that the igneous complex may be related to the numerous amphibolite units, which are intercalated with the metapelites of the overlying Kinzigite Formation, and together with them may constitute an accretionary complex. In this case, the mafic-ultramafic complex itself might also be part of such an accretionary complex (as has been proposed for the Balmuccia peridotite). Internal Sm-Nd isochrons involving grt, cpx, plag and amph from the Internal Gabbro unit yield concordant ages of 231 ± 23, 226 ± 7, 223 ± 10, 214 ± 17, and 203 ± 13 Ma. These results confirm published evidence for a separate, regional heating event about 215 ± 15 Ma ago. Initial Nd(533) values average +6.3 ± 0.4 for six samples of the Amphibole Peridotite unit and +6.0 ± 1.2 for ten samples of the External Gabbro unit. 87Sr/86Sr ratios require little or no age correction and range from 0.7026 to 0.7047 (with two outliers at 0.7053 and 0.7071). Strong correlations between 87Sr/86Sr and K2O and weaker correlations between initial Nd and K2O imply a comparatively minor (≤ 10%) contamination of the External Gabbro magma by crustal material and a later alteration by a crustal or seawater-derived fluid. These results contrast sharply with the isotopic composition (negative Nd and high 87Sr/86Sr values) of the associated mantle rocks, the Phlogopite Peridotite unit, which has been pervasively metasomatized by crustal fluids. This type of metasomatism and its isotopic signature are never seen in the magmatic complex. This evidence rules out any direct genetic relationship between the igneous complex and the mantle peridotite. The crust-mantle interaction is the opposite of that seen at Balmuccia, where the mantle peridotite is essentially ‘pristine’ and the magmatic body has been extensively contaminated by assimilation of crustal rocks. 相似文献
17.
The Oshurkovo Complex is a plutonic sheeted complex which represents numerous successive magmatic injections into an expanding system of subparallel and subvertical fractures. It comprises a wide range of rock types including alkali monzodiorite, monzonite, plagioclase-bearing and alkali-feldspar syenites, in the proportion of about 70% mafic rocks to 30% syenite. We suggest that the variation within the complex originated mainly by fractional crystallization of a tephrite magma. The mafic rocks are considered as plutonic equivalents of lamprophyres. They exhibit a high abundance of ternary feldspar and apatite, the latter may attain 7–8 vol.% in monzodiorite. Ternary feldspar is also abundant in the syenites. The entire rock series is characterized by high Ba and Sr concentrations in the bulk rock samples (3000–7000 ppm) and in feldspars (up to 1 wt.%). The mafic magma had amphibole at the liquidus at 1010–1030 °C based on amphibole geothermometer. Temperatures as low as this were due to high H2O and P2O5 contents in the melt (up to 4–6 and 2 wt.%, respectively). Crystallization of the syenitic magmas began at about 850 °C (based on ternary feldspar thermometry). The series was formed at an oxygen fugacity from the NNO to HM buffer, or even higher. The evolution of the alkali monzodiorite–syenite series by fractional crystallization of a tephritic magma is established on the basis of geological, mineralogical, geochemical and Sm–Nd and Rb–Sr isotope data. The geochemical modeling suggests that fractionation of amphibole with subordinate apatite from the tephrite magma leaves about 73 wt.% of the residual monzonite melt. Further extraction of amphibole and plagioclase with minor apatite and Fe–Ti oxides could bring to formation of a syenite residuum. Rb–Sr isotopic analyses of biotite, apatite and whole-rock samples constrain the minimum age of basic intrusions at ca. 130 Ma and that of cross-cutting granite pegmatites at ca. 120 Ma. Hence the entire evolution took place in an interval of ≤10 My. Initial 87Sr/86Sr ratios for the mafic rocks range from 0.70511 to 0.70514, and for syenites from 0.70525 to 0.70542. Initial Nd (130 Ma) values for mafic rocks vary from −1.9 to −2.4, and for syenites from −2.9 to −3.5. In a Nd(T) vs. (87Sr/86Sr)i diagram, all rock types of the complex fall in the enriched portion of the Mantle Array, suggesting their derivation from a metasomatized mantle source. However, the small but distinguishable difference in Sr and Nd isotopic compositions between mafic rocks and syenites probably resulted from mild (10–20%) crustal contamination during differentiation. Large negative Nb anomalies are interpreted as a characteristic feature of the source region produced by Precambrian fluid metasomatism above a subduction zone rather than by crustal contamination. 相似文献
18.
The Korosten complex is a Paleoproterozoic gabbro–anorthosite–rapakivi granite intrusion which was emplaced over a protracted time interval — 1800–1737 Ma. The complex occupies an area of about 12 000 km 2 in the north-western region of the Ukrainian shield. About 18% of this area is occupied by various mafic rocks (gabbro, leucogabbro, anorthosite) that comprise five rock suites: early anorthositic A 1 (1800–1780 Ma), main anorthositic A 2 (1760 Ma), early gabbroic G 3 (between 1760 and 1758 Ma), late gabbroic G 4 (1758 Ma), and a suite of dykes D 5 (before 1737 Ma). In order to examine the relationships between the various intrusions and to assess possible magmatic sources, Nd and Sr isotopic composition in mafic whole-rock samples were measured. New Sr and Nd isotope measurements combined with literature data for the mafic rocks of the Korosten complex are consistent and enable construction of Rb–Sr and Sm–Nd isochronous regressions that yield the following ages: 1870 ± 310 Ma (Rb–Sr) and 1721 ± 90 Ma (Sm–Nd). These ages are in agreement with those obtained by the U–Pb method on zircons and indicate that both Rb–Sr and Sm–Nd systems have remained closed since the time of crystallisation. In detail, however, measurable differences in isotopic composition of the Korosten mafic rock depending on their suite affiliation were revealed. The oldest, A 1 rocks have lower Sr ( 87Sr/ 86Sr (1760) = 0.70233–0.70288) and higher Nd ( εNd (1760) = 1.6–0.9) isotopic composition. The most widespread A 2 anorthosite and leucogabbro display higher Sr and lower Nd isotopic composition: 87Sr/ 86Sr (1760) = 0.70362, εNd (1760) varies from 0.2 to − 0.7. The G 3 gabbro–norite has slightly lower εNd (1760) varying from − 0.7 to − 0.9. Finally, G 4 gabbroic rocks show relatively high initial 87Sr/ 86Sr (0.70334–0.70336) and the lowest Nd isotopic composition ( εNd (1760) varies from − 0.8 to − 1.4) of any of the mafic rocks of the Korosten complex studied to date. On the basis of Sr and Nd isotopic composition we conclude that Korosten initial melts may have inherited their Nd and Sr isotopic characteristics from the lower crust created during the 2.05–1.95 Ga Osnitsk orogeny and 2.0 Ga continental flood basalt event. Indeed, εNd (1760) values in Osnitsk rocks vary from 0.0 to − 1.9 and from 0.2 to 3.4 in flood basalts. We suggest that these rocks being drawn into the upper mantle might melt and give rise to the Korosten initial melts. 87Sr/ 86Sr (1760) values also support this interpretation. We suggest that the Sr and Nd isotopic data currently available on mafic rocks of the Korosten complex are consistent with an origin of its primary melts by partial melting of lower crustal material due to downthrusting of the lower crust into upper mantle forced by Paleoproterozoic amalgamation of Sarmatia and Fennoscandia. 相似文献
19.
Calc-alkaline magmatism in the south-west Ukraine occurred between 13.8 and 9.1 Ma and formed an integral part of the Neogene subduction-related post-collisional Carpathian volcanic arc. Eruptions occurred contemporaneously in two parallel arcs (here termed Outer Arc and Inner Arc) in the Ukrainian part of the Carpathians. Outer Arc rocks, mainly andesites, are characterized by LILE enrichment (e.g. K and Pb), Nb depletion, low compatible trace element abundances, high 87Sr/ 86Sr, high δ 18O and low 143Nd/ 144Nd isotopic ratios (0.7085–0.7095, 7.01–8.53, 0.51230–0.51245, respectively). Inner Arc rocks are mostly dacites and rhyolites with some basaltic and andesitic lavas. They also show low compatible element abundances but have lower 87Sr/ 86Sr, δ 18O and higher 143Nd/ 144Nd ratios (0.7060–0.7085, 6.15–6.64, 0.5125–0.5126, respectively) than Outer Arc rocks. Both high-Nb and low-Nb lithologies are present in the Inner Arc. Based on the LILE enrichment (especially Pb), a higher fluid flux is suggested for the Outer Arc magmas compared with those of the Inner Arc. Combined trace element and Sr–Nd–O isotopic modelling suggests that the factors which controlled the generation and evolution of magmas were complex. Compositional differences between the Inner and Outer Arcs were produced by introduction of variable proportions of slab-derived sediments and fluids into a heterogeneous mantle wedge, and by different extents of upper crustal contamination. Degrees of magmatic fractionation also differed between the two arcs. The most primitive magmas belong to the Inner Arc. Isotopic modelling shows that they can be produced by adding 3–8% subducted terrigenous flysch sediments to the local mantle wedge source. Up to 5% upper crustal contamination has been modelled for fractionated products of the Inner Arc. The geochemical features of Outer Arc rocks suggest that they were generated from mantle wedge melts similar to the Inner Arc primitive magmas, but were strongly affected by both source enrichment and upper crustal contamination. Assimilation of 10–20% bulk upper crust is required in the AFC modelling, assuming an Inner Arc parental magma. We suggest that magmagenesis is closely related to the complex geotectonic evolution of the Carpathian area. Several tectonic and kinematic factors are significant: (1) hydration of the asthenosphere during subduction and plate rollback directly related to collisional processes; (2) thermal disturbance caused by ascent of hot asthenospheric mantle during the back-arc opening of the Pannonian Basin; (3) clockwise translational movements of the Intracarpathian terranes, which facilitated eruption of the magmas. 相似文献
20.
Late Neoproterozoic bimodal dyke suites are abundant in the Arabian–Nubian Shield. In southern Israel this suite includes dominant alkaline quartz porphyry dykes, rare mafic dykes, and numerous composite dykes with felsic interiors and mafic margins. The quartz porphyry chemically corresponds to A-type granite. Composite dykes with either abrupt or gradational contacts between the felsic and mafic rocks bear field, petrographic and chemical evidence for coexistence and mixing of basaltic and rhyolitic magmas. Mixing and formation of hybrid intermediate magmas commenced at depth and continued during emplacement of the dykes. Oxygen isotope ratios of alkali feldspar in quartz porphyry (13 to 15‰) and of plagioclase in trachydolerite (10–11‰) are much higher than their initial magmatic ratios predicted by equilibrium with unaltered quartz (8 to 9‰) and clinopyroxene (5.8‰). The elevation of δ18O in alkali feldspar and plagioclase, and extensive turbidization and sericitization call for post-magmatic low-temperature (≤ 100 °C) water–rock interaction. Hydrous alteration of alkali feldspar, the major carrier of Rb and Sr in the quartz–porphyry, also accounts for the highly variable and unusually high I(Sr) of 0.71253 to 0.73648. The initial 143Nd/144Nd ratios, expressed by εNd(T) values, are probably unaltered and show small variation in mafic and felsic rocks within a narrow range from + 1.4 to + 3.3. The Nd isotope signature suggests either a common mantle source for the mafic and silicic magmas or a juvenile crustal source for the felsic rocks (metamorphic rocks from the Elat area). However, oxygen isotope ratios of zircon in quartz porphyry [δ18O(Zrn) = 6.5 to 7.2‰] reveal significant crustal contribution to the rhyolite magma, suggesting that mafic and A-type silicic magmas are not co-genetic, although coeval. Comparison of 18O/16O ratios in zircon allows to distinguish two groups of A-type granites in the region: those with mantle-derived source, δ18O(Zrn) ranging from 5.5 to 5.8‰ (Timna and Katharina granitoids) and those with major contribution of the modified juvenile crustal component, δ18O(Zrn) varying from 6.5 to 7.2‰ (Elat quartz porphyry dykes and the Yehoshafat alkaline granite). This suggests that A-type silicic magmas in the northern ANS originated by alternative processes almost coevally. 相似文献
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