Granulite and pyroxenite xenoliths in lamprophyre dykes intruded during the waning stage of Deccan Trap volcanism are derived from the lower crust beneath the Dharwar craton of Western India. The xenolith suite consists of plagioclase-poor mafic granulites (55% of the total volume of xenoliths), plagioclase-rich felsic granulites (25%), and ultramafic pyroxenites and websterites (20%) with subordinate wehrlites. Rare spinel peridotite xenoliths are also present, representing mantle lithosphere. The high Mg #, low SiO2/Al2O3 and low Nb/La (<1) ratios suggest that the protoliths of the mafic granulites broadly represent cumulates of sub-alkaline magmas. All of the granulites are peraluminous and light rare-earth element-enriched. The felsic granulites may have resulted from anatexis of the mafic lower crustal rocks; thus, the mafic granulites are enriched in Sr whereas the felsic ones are depleted. Composite xenoliths consisting of mafic granulites traversed by veins of pyroxenite indicate intrusion of the granulitic lower crust by younger pyroxenites. Petrography and geochemistry of the latter (e.g. presence of phlogopite) indicate the metasomatised nature of the deep crust in this region.Thermobarometric estimates from phase equilibria indicate equilibration conditions between 650 and 1200 °C, 0.7-1.2 GPa suggestive of lower crustal environments. These estimates provide a spatial context for the sampled lithologies thereby placing constraints on the interpretation of geophysical data. Integration of xenolith-derived P-T results with Deep Seismic Soundings (DSS) data suggests that the pyroxenites and websterites are transitional between the lower crust and the upper mantle. A three-layer model for the crust in western India, derived from the xenoliths, is consistent with DSS data. The mafic nature of this hybrid lower crust contrasts with the felsic lower crustal composition of the south Indian granulite terrain. 相似文献
Clinopyroxenes from pyroxenite, ijolite and nepheline syenite from the main intrusion of the Alnö complex define two sub-parallel compositional trends with respect to Na, Ca and FeTOT plotted against alkali-pyroxene fractionation index (Na–Mg). Both trends define a smooth fractionation of increasing Na and FeTOT and decreasing Ca with increasing Na–Mg, but one set of samples contain clinopyroxenes that constantly plot at higher Na and lower FeTOT and Ca (at similar Na–Mg) than the rest of the samples. Clinopyroxenes with higher Ca and FeTOT and lower Na (trend 1) co-exist with substantial amounts of Ti-andradite (up to 70 vol.%), while the sample set defining the more Na-rich trend (trend 2) lack co-existing Ti-andradite. Clinopyroxenes from both trends show fractionated REE patterns with a distinct difference in HREE content, reflecting the content of co-existing Ti-andradite. The rocks of the first Ti-andradite-bearing trend crystallized slightly prior to the rocks of the second trend, probably from a primitive, Ca- and Ti-rich nephelinitic magma. Crystallisation of pyroxenite and melteigite occurred under low aSiO2 and high aCaO and aTiO2 as evidenced by the presence of perovskite and sometimes substantial amounts of magnetite. Subsequent increase in aSiO2 is evidenced in the overgrowth of perovskite by titanite, which in turn is overgrown by Ti-andradite. Nepheline syenitic residuals crystallized under higher aSiO2 and aNa2O and lower aCaO and aTiO2, which reduced Ti-andradite into an accessory phase and produced more Si- and Na-rich clinopyroxenes. Some of these residuals probably also mixed with new primitive magma producing a hybrid magma that crystallised the more Na-rich and Ca- and FeTOT-poor clinopyroxenes of trend 2. The complete lack of Ti-andradite in these rocks indicates different crystallisation conditions and also a different magma composition. 相似文献
Several types of xenoliths occur in a Permian basanite sill in Fidra, eastern central Scotland. One group consists of spinel lherzolites, which have geochemical and isotopic characteristics similar to those of lithospheric upper mantle from elsewhere in western Europe, with both LREE-depleted and LREE-enriched compositions. A separate group comprises pyroxenites and wehrlites, some of which contain plagioclase; these have compositions and textures that indicate that they are cumulates from mafic magmas. In terms of Sr and Nd isotope compositions, the pyroxenites closely resemble the host basanite and most likely formed by high-pressure fractionation of Permo-Carboniferous alkaline magmas at lower crustal depths. They also have mantle-like δ18O values. A third group is composed of granulite xenoliths that vary between plagioclase-rich and clinopyroxene-rich compositions, some of which probably form a continuum with the pyroxenites and wehrlites. They are all LREE-enriched and most have positive Eu anomalies; thus, they are also mostly cumulates from mafic magmas. Many of the granulites also have Sr and Nd radiogenic isotope ratios similar to those of the host basanite, indicating that they have formed from a similar magma. However, several of the granulites show more enriched isotopic compositions, including higher δ18O values, trending towards an older crustal component. Thus, the pyroxenites and granulites are largely cogenetic and are mainly the product of a mafic underplating event that occurred during the widespread magmatism in central Scotland during Permo-Carboniferous times. 相似文献
Widespread magmatic activity developed in the Middle Miocene in the Cappadocian Region of Central Anatolia in Turkey. Despite several previous studies that focus on the geochemical features of the magmatic rocks, the source components and development of melting conditions are still a matter of debate.Recent basaltic rocks from Karaburna and Gül?ehir (1228 and 96 Ka, respectively, Dogan, 2011) are considered as a part of the Central Anatolian Volcanic Province, situated at the northernmost end of the Cappadocian Region. These lavas have similar large ion lithophile (LIL) (except Rb) and high field strength (HFS) element abundances, however, Karaburna samples are more enriched in HFS elements, and both of the rocks suites reflect HFS depletions relative to the OIB signature.Karaburna and Gül?ehir basalts have low Nb/La (0.45–0.5; 0.35–0.5), Nb/Th (2.75–4.61; 1.26–2.85) values, respectively, suggesting contributions from crustal sources, whereas Zr/Ba ratios of these samples range between 0.32–0.93 and 0.4–0.88 and imply that these rocks appear to be derived from asthenospheric sources. These incompatible element ratios can be attributed to either different geochemical processes, or are related to melting from different source component(s).The ambient mantle source of the Cappadocian region appears to be consistent with spinel peridotite, but this domain is not solely satisfactory to represent the melting conditions in the light of new elemental data. Values of Tb/Yb(N) and Zn/Fe provide new constraints suggesting the magmas were generated from the asthenosphere. Tb/Yb(N) ratio separates garnet – spinel transition Tb/Yb(N) (>1.8) and Zn/Fe ratio displays separation between the peridotite-derived (Zn/Fe <12) and pyroxenite-derived (13?20) melts.A melting model based on REE ratios and Zn/Fe, Co/Fe, Tb/Yb(N) values indicates that basaltic rocks were not derived from a single source component (peridotite). Instead, those values suggest substantial melting contributions from a pyroxenite source domain, which has not been discussed as a source component in previous studies. Melts, from both of the source domains, with the result of asthenospheric upwelling linked to the downgoing Aegean and Cyprean slabs, are distinct from the alkaline character frequently observed as the final products of recent volcanic activity in the Cappadocian region and also explains the different trace element variations observed in such a small scale. 相似文献
Garnet-bearing and garnet-free pyroxenite xenoliths from Quaternary basanites of Marsabit, northern Kenya, were analysed for
microstructures and mineral compositions (major and trace elements) to constrain the thermal and compositional evolution of
the lithospheric mantle in this region. Garnet-bearing rocks are amphibole-bearing websterite with ~5–10 vol% orthopyroxene.
Clinopyroxene is LREE-depleted and garnet has high HREE contents, in agreement with an origin as cumulates from basaltic mantle
melts. Primary orthopyroxene inclusions in garnet suggest that the parental melts were orthopyroxene-saturated. Rock fabrics
vary from weakly to strongly deformed. Thermobarometry indicates extensive decompression and cooling (~970–1,100°C at ~2.3–2.6 GPa
to ~700–800°C at ~0.5–1.0 GPa) during deformation, best interpreted as pyroxenite intrusion into thick Paleozoic continental
lithosphere subsequently followed by continental rifting (i.e., formation of the Mesozoic Anza Graben). During continental
rifting, garnet websterites were decompressed (garnet-to-spinel transition) and experienced the same P–T evolution as their host peridotites. Strongly deformed samples show compositional overlaps with cpx-rich, initially garnet-bearing
lherzolite, best explained by partial re-equilibration of peridotite and pyroxenite during deformation and mechanical mingling.
In contrast, garnet-free pyroxenites include undeformed, cumulate-like samples, indicating that they are younger than the
garnet websterites. Major and trace element compositions of clinopyroxene and calculated equilibrium melts suggest crystallisation
from alkaline basaltic melt similar to the host basanite, which suggests formation in the context of alkaline magmatism during
the development of the Kenya rift.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
The compositional variability of the lithospheric mantle at extensional settings is largely caused by the reactive percolation of uprising melts in the thermal boundary layer and in lithospheric environments.The Alpine-Apennine(A-A)ophiolites are predominantly constituted by mantle peridotites and are widely thought to represent analogs of the oceanic lithosphere formed at ocean/continent transition and slow-to ultraslow-spreading settings.Structural and geochemical studies on the A-A mantle peridotites have revealed that they preserve significant compositional and isotopic heterogeneity at variable scale,reflecting a long-lived multi-stage melt migration,intrusion and melt-rock interaction history,occurred at different lithospheric depths during progressive uplift.The A-A mantle peridotites thus constitute a unique window on mantle dynamics and lithosphere-asthenosphere interactions in very slow spreading environments.In this work,we review field,microstructural and chemical-isotopic evidence on the major stages of melt percolation and melt-rock interaction recorded by the A-A peridotites and discuss their consequences in creating chemical-isotopic heterogeneities at variable scales and enhancing weakening and deformation of the extending mantle.Focus will be on three most important stages:(i)old(pre-Jurassic)pyroxenite emplacement,and the significant isotopic modification induced in the host mantle by pyroxenite-derived melts,(ii)melt-peridotite interactions during Jurassic mantle exhumation,i.e.the open-system reactive porous flow at spinel facies depths causing bulk depletion(origin of reactive harzburgites and dunites),and the shallower melt impregnation which originated plagioclase-rich peridotites and an overall mantle refertilization.We infer that migrating melts largely originated as shallow,variably depleted,melt fractions,and acquired Si-rich composition by reactive dissolution of mantle pyroxenes during upward migration.Such melt-rock reaction processes share significant similarities with those documented in modern oceanic peridotites from slow-to ultraslow-spreading environments and track the progressive exhumation of large mantle sectors at shallow depths in oceanic settings where a thicker thermal boundary layer exists,as a consequence of slow-spreading rate. 相似文献
The Maowu eclogite–pyroxenite body is a small (250×50 m) layered intrusion that occurs in the ultra-high-pressure (UHP) metamorphic terrane of Dabieshan, China. Like the adjacent Bixiling complex, the Maowu intrusion was initially emplaced at a crustal level, then subducted along with the country gneisses to mantle depths and underwent UHP metamorphism during the collision of the North and South China Blocks in the Triassic. This paper presents the results of a geochemical and isotopic investigation on the metamorphosed Maowu body. The Maowu intrusion has undergone open system chemical and isotopic behavior three times. Early crustal contamination during magmatic differentiation is manifested by high initial 87Sr/86Sr ratios (0.707–0.708) and inhomogeneous negative Nd(T) values of −3 to −10 at 500 Ma (probable protolith age). Post-magmatic and pre-UHP metamorphic metasomatism is indicated by sinusoidal REE patterns of garnet orthopyroxenites, lack of whole-rock (WR) Sm–Nd isochronal relationship, low δ18O values and an extreme enrichment of Th and REE in a clinopyroxenite. Finally, K and Rb depletion during UHP metamorphism is deduced from the high initial 87Sr/86Sr ratios unsupported by in situ Rb/Sr ratios. Laser ICP-MS spot analyses on mineral grains show that (1) Grt and Cpx attained chemical equilibrium during UHP metamorphism, (2) Cpx/Grt partition coefficients for REE correlate with Ca, and (3) LREE abundances in whole rocks are not balanced by that of the principal phases (Grt and Cpx), implying that the presence of LREE-rich accessory phases, such as monazite and apatite, is required to account for the REE budget.
Sm–Nd isotope analyses of minerals yielded three internal isochrons with ages of 221±5 Ma and (T)=−5.4 for an eclogite, 231±16 Ma and (T)=−6.2 for a garnet websterite, and 236±19 Ma and (T)=−6.9 for a garnet clinopyroxenite. The Cpx/Grt chemical equilibrium and the consistent mineral isochron ages indicate that the metasomatic processes mentioned above must have occurred prior to the UHP metamorphism. These Sm–Nd ages agree with published zircon and monazite U–Pb ages and constrain the time of UHP metamorphism to 220–236 Ma. The Maowu and Bixiling layered intrusions are similar in their in situ tectonic relationship with their country gneisses, but the two bodies are distinguished by their magma-chamber processes. The Bixiling magmas were contaminated by the lower crust, whereas the Maowu magmas were contaminated by the upper crustal rocks during their emplacement and differentiation. The two complexes represent two distinct suites of magmatic rocks, which have resided in the continental crust for about 300–400 Ma before their ultimate subduction to mantle depths, UHP metamorphism and return to the crustal level. 相似文献