We have obtained major and trace element data for a suite of rocks emplaced over an area of 45,000 km2 in the Eastern Goldfields Province (EGP), Yilgarn Craton, that are petrographically and mineralogically described as kimberlites, melnoites and carbonatites. Kimberlites dominate the rock types found in the west whereas carbonatites and melnoites are common in the east. Compatible element data from the carbonatites and melnoites tend to lie along trends that imply silicate–carbonate fractionation. The kimberlites exhibit a much greater amount of compatible element scatter as a result of the variable contribution from continental lithospheric mantle (CLM). When compared to southern African kimberlites, the EGP kimberlites have consistently lower MgO and Os concentrations at comparable Ni concentrations. The opposite is true for Ti–Ni variation where the EGP kimberlites have higher Ti than the southern African kimberlites at comparable Ni concentrations. These data are interpreted to suggest that the CLM beneath the province was either melt metasomatised some time prior to kimberlite emplacement, or that the EGP CLM is less refractory (melt depleted) than the Kaaapvaal CLM.
In contrast, the incompatible element ratios and initial Nd values are constant throughout the entire rock suite. Carbonate C and O isotope data show a broad positive correlation, consistent with magmatic-hydrothermal trends found in many carbonatite complexes. These incompatible element and isotope data link all of the rocks within the province to the same mantle source that was similar to modern-day mantle plume sources.
Re–Os data for the various samples, including oxide minerals from all of the petrographic types, xenocryst-poor kimberlites and melnoites yield a precise Re–Os isochron of 2025±10 Ma and an initial γOs of 5.3±3.1 (MSWD=5.8). These data support the conclusion based on incompatible element, stable isotope and Sm–Nd isotope data that the rocks are comagmatic.
Initial Os isotopic compositions and Re/Os ratios for the xenocryst-rich kimberlites are also correlated. However, the correlation does not pass through the calculated initial γOs and Re/Os of the isochron. The Re–Os data show that the incompatible element-enriched melt exhibited very little control on the Re–Os variations of the xenocryst-rich kimberlites.
Correlations between deep mantle seismic velocities, petrology and whole-rock compatible element geochemistry suggest that the rheology and history of the EGP CLM played a significant role in determining the petrographic characteristics of the magmas that were ultimately emplaced into the EGP crust. 相似文献
The granitic dykes in the Badu Group,Zhejiang Province,South China provide important insights on tectonic setting and crustal evolution of the South China Block(SCB) and the Indochina Block during Triassic.Here we report LA-ICP-MS U-Pb data of granitic rocks from the Hucun and Kengkou which show early Triassic ages of 242 ± 2 and 232 ± 3 Ma,respectively,representing their timing of emplacement.The dyke rocks are enriched in K,Al,LREE,Rb,Th.U,and Pb.and are depleted in Nb,Ta,Sr,and Ti.The rocks are characterized by highly fractionated REE patterns with(La/Yb)N ratios of 28.46-38.07 with strong negative Eu anomalies(Eu/Eu* = 0.65-0.73).In situ Hf isotopic analyses of zircons from the Hucun granite yielded ε_(Hf)(t) values of-13.9 to-6.4 and two-stage depleted mantle Hf model ages of 1.68-2.15 Ga,which indicate that the magma was formed by partial melting of the Paleoproterozoic metasedimentary protoliths in the Cathaysia Block.The zircons from the Kengkou granite have ε_(Hf)(t) values ranging from 40.7 to 31.5 and yield two-stage depleted mantle Hf model ages of 0.99-2.49 Ga,indicating magma origin from a mixed source.The Hucun and Kengkou dykes,together with the Triassic A-type granites in SE China were probably generated during magmatism associated with crust-mantle decoupling along the convergent plate boundary between SCB and the Indochina Block. 相似文献
The 50 km2 Monywa copper district lies near the Chindwin River within the northward continuation of the Sunda‐Andaman magmatic arc through western Myanmar. There are four deposits; Sabetaung, Sabetaung South, Kyisintaung, and the much larger Letpadaung 7 km to the southeast. Following exploration drilling which began in 1959, production of copper concentrates from a small open pit started at Sabetaung in 1983. Since 1997, when resources totaled 7 million tonnes contained copper in 2 billion tonnes ore, a heap leach–electro‐winning operation has produced over 400,000 t copper cathode from Sabetaung and Sabetaung South. Ore is hosted by mid‐Miocene andesite or dacite porphyry intrusions, and by early mid‐Miocene sandstone and overlying volcaniclastics including eruptive diatreme facies which the porphyries intrude. District‐wide rhyolite dykes and domes with marginal breccias probably post‐date andesite porphyries in the mine area and lack ore‐grade copper. Host rocks to mineralization are altered to phyllic and advanced argillic hydrothermal assemblages within an outer chlorite zone; hypogene alunite is most abundant at Letpadaung and Kyisintaung. Most mineralization is structurally‐controlled with digenite‐chalcocite in breccia dykes, in steeply dipping NE‐trending sheeted veins, and in stockwork and low‐angle sulfide veins. A high‐grade pipe at Sabetaung grades up to 30% Cu, and much of the ore at Sabetaung South is in a NE‐trending zone of mega‐breccia and stockworked sandstone. The hydrothermal alteration, together with replacement quartz, alunite and barite in breccia dykes and veins, the virtual absence of vein quartz, and the presence of chalcopyrite and bornite only as rare veins and as inclusions within the abundant pyrite, indicate that the deposits are high sulfidation. Regional uplift, resistance to erosion and leaching of the altered and mineralized rocks have resulted in porous limonite‐stained leached caps over 200 m thick forming the Letpadaung and Kyisintaung hills. The barren caps pass abruptly downwards at the water table into the highest grade ore at the top of the supergene enrichment zone, within which copper grade, supergene kaolinite and cubic alunite decrease, and pyrite increases with depth; in contrast, marcasite is mostly shallow. Much of the copper to depths exceeding 200 m below the water table occurs as supergene digenite‐chalcocite and minor covellite. Disseminated chalcocite is mostly near‐surface and hence almost certainly supergene. We infer that during prolonged uplift at all four deposits, oxidation of residual pyrite at the water table generated enough acid to leach all the copper from earlier supergene‐enriched ore; below the water table the resulting acid sulfate solutions partly replaced enargite, covellite, chalcopyrite, bornite and pyrite with supergene chalcocite. Undeformed upward‐fining cross‐bedded conglomerates and sands of the ancestral Chindwin River floodplain overlie the margins of the Sabetaung deposits, form a major aquifer up to 40 m thick, and are a potential host for exotic copper mineralization. A mid‐Miocene pluton is inferred to underlie the Monywa deposits, but the possibility of porphyry‐type mineralization within the district is at best highly speculative. 相似文献
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
Fifteen zircons separated from a mafic dyke in the Chinese Altai give a concordant age population with a weighted mean 206Pb/238U age of 375.5 ± 4.8 Ma, suggesting a Devonian emplacement. On the basis of their mineralogical compositions and textures, the coeval dykes can be divided into gabbroic and doleritic types. They are both sub-alkaline, tholeiitic, characterized by similarly low SiO2 contents (45.2–52.7 wt.%) and total alkaline (K2O + Na2O = 0.99–4.93 wt.%). Rare earth element patterns of the gabbroic dykes are similar to N-MORB (La/YbN = 0.86–1.1), together with their high εNd(t) values (+ 7.6 to + 8.1), indicating that their precursor magma was mainly derived from a N-MORB-type depleted asthenospheric mantle. While the REE patterns of the doleritic dykes resemble that of E-MORB (La/YbN = 1.12–2.28), enriched in LILEs and strongly depleted in HFSEs, with relative low εNd(t) values (+ 3.4 to + 5.4) and high initial 87Sr/86Sr ratios (0.7057–0.7060). The zircon Hf isotopic analysis of the doleritic dykes give εHf(t) values from + 10.7 to + 13.8. These signatures suggest that a depleted mantle wedge metasomatized by slab-derived fluids and/or melts was possibly involved in the generation of the doleritic magma. The refractory peridotite may have been melted with variable degrees caused by upwelling of the hot asthenosphere. The petrogenesis of the mafic dykes suggest a high heat flux as a result of upwelling of the hot asthenosphere and the contrast geochemical signatures can be interpreted by a ridge subduction, which could be an important tectonic control in the accretionary process of the Chinese Altai. 相似文献