Early Tertiary volcanic rocks recovered from the southeast Greenland margin represent the transition from continental tholeiitic flood basalt to voluminous oceanic magmatism.This magmatism accompanied continental breakup and resulted in the formation of seaward-dipping reflector sequences (SDRS) characteristic of volcanic rifted margins.The earliest, and most landward, lava flows (Hole 917A, Lower and Middle Series) comprise a pre-breakup continental sequence ranging in composition from olivine basalt to dacite.Evolution in crustal magma reservoirs, with a dwindling supply of primitive magma and an increasing role of crustal contamination, can account for the variations in magma composition.Changes in the inferred nature of the contaminant suggest that the site of magma storage may have moved to shallower levels in the crust during the prebreakup period.The subsequent eruption of picrite and olivine basalt magmas (Hole 917A, Upper Series) marked a dramatic change in the style of magmatism to one of unrestrained passage of primitive magma from the mantle to the surface during the final stages of breakup.The younger parts of the SDRS (Sites 915 and 918) are composed of compositionally uniform basalt (7.6 ± 0.8% MgO), suggesting that an effective magmatic filtering system was established, soon after breakup, in magma chambers associated with a spreading axis.An increase in degree of mantle melting (<5% to 10%-20%), accompanied by a decrease in depth of melt segregation, marked the transition from continental to oceanic volcanism.The continental volcanic rocks had a garnet lherzolite source, whereas the post-breakup magmas had a shallower, spinel lherzolite source.Most of the older basalts (Site 917, Lower to Upper Series) had a mantle source similar to that of normal mid-ocean-ridge basalt (N-MORB), although a group of flows with compositions similar to typical Icelandic basalt occurs high in the Lower Series.All the post-breakup basalt lava flows (Sites 915 and 918) had a depleted Icelandic mantle source.The head of the Iceland plume may have been zoned at the time of continental breakup with a core of Icelandic mantle surrounded by a carapace of anomalously hot N-MORB-source mantle.
Summary The mafic lavas of the Azores and Cape Verde Islands are of a highly varied composition, reflecting a complex history of magma genesis and a variety of source compositions. The lavas of the Cape Verde Islands are characteristically highly silica undersaturated, with alkali-rich ankaramites, larnite-normative melilitites and carbonatites. In contrast, the lavas of the Azores vary from strongly nepheline- to hypersthene-normative types. Isotopic ratios and trace elements also show considerable variation, consistent with derivation from multicomponent mantle sources. Three distinct groupings of lava compositions can be seen in Pb-Sr-Nd isotope space, and each is characterized by its own trace-element enrichment. (i) The majority of the lavas from the Azores and northern Cape Verdes Islands have isotope and trace-element systematics similar to basalts recovered from elevated segments of the mid-Atlantic Ridge ( eg during Deep Sea Drilling Project Leg 82) (Ba/La <12, 206 Pb/ 204 Pb=19.3–20.0). (ii) The islands of Sao Miguel and Faial in the Azores are characterized by radiogenic strontium and lead isotope ratios and marked fractionation of the most incompatible trace elements ( eg 87 Sr/ 86 Sr=0.70522, 206 Pb/ 204 Pb=19.88, 207 Pb/ 204 Pb=15.75, Ba/Nb=12, La/Nb=0.84). (iii) In contrast, the southern Cape Verde Islands have relatively unradiogenic Pb-Sr-Nd isotope ratios ( eg 87 Sr/ 86 Sr=0.70393, 206 Pb/ 204 Pb=18.74, 207 Pb/ 204 Pb=15.53, Ba/La=22, Ba/Nb=15, La/Nb=0.67). Mixing relationships between isotope and trace-element ratios indicate the involvement of at least four, and possibly five, chemically distinct source regions in the petrogenesis of mafic lavas. It is shown that these different source regions represent combinations of the depleted upper mantle (mid-ocean ridge basalt source), recycled oceanic lithosphere and two components of the subcontinental lithospheric mantle. No direct unequivocal evidence is found for a fifth component, namely undepleted primitive mantle reservoir, although helium isotope data suggest influxes of material and heat from the lower mantle. Similarly, it is suggested that continental crust has no direct contribution to the petrogenesis of these ocean island basalts and its role in the mantle is limited to the production/modification of the subcontinental lithosphere above subduction zones.
SUMMARY In the Soom Shale, labile soft tissues have been replaced rapidly after death by authigenic clay minerals which now have an illitic composition. There are two possible pathways for this mineralization: (1) initial replacement of soft tissue by kaolinite which was then diagenetically transformed to illite; or (2) direct replacement of soft tissue by illite. Comparison of the compositions of the authigenic illites that replace the soft tissues and of the detrital illites in the host rock show that the former have higher magnesium numbers and enhanced potassium contents. These compositional differences are better explained if illite directly replaced the soft tissues. Kaolinite and/or illite stability are controlled by the pH and the ratio of K+/H+ in the fluid, expressed as: 2(KAl 3 Si 3 O 10 (OH) 2 ) + 3H 2 O + 2H + ↔ 3(Al 2 Si 2 O 5 (OH) 4 ) + 2K + . Although the porewaters of the Soom Shale were anoxic and, at times, euxinic, and so might well have favoured kaolinite authigenesis, the conditions within the local environment of the carcasses could have been very different. Ammonia, produced by protein decay, and potassium, concentrated within the tissues, would have been elevated relative to the surrounding pore waters, so that the K + /H + ratio would have favoured illite authigenesis. Authigenic illites have higher magnesium numbers than detrital sedimentary illites owing to extensive and early pyrite precipitation, indicated by small pyrite framboid diameters (average 4.3 μ), which left the porewaters depleted in iron. The magnesium number in authigenic illites could also have been enhanced by the inclusion of magnesium from seawater.
Whole-rock geochemical data were obtained for 178 samples selected from the petrologically freshest parts of most of the flow units recovered from the nine holes.These data include 170 major element analyses (XRF) and 174 trace-element analyses (XRF and INAA).We include representative microprobe analyses of the primary minerals and fresh basaltic glasses (where present) in 40 selected samples.We also present 87 Sr/ 86 Sr ratios for 14 samples selected from the holes at 63°N.These analytical data, together with petrographic data obtained from the study of some 500 thin sections, have enabled us to establish a detailed lithostratigraphy for each basement section.
Abstract The Bhagirathi leucogranite forms a series of low-angle en echelon, lensoidal intrusions at the top of the High Himalayan slab in the central Himalaya of Garhwal, northern India. The leucogranite comprises the assemblage: K-feldspar + quartz + plagioclase + tourmaline + muscovite ± biotite ± garnet. Compared to other High Himalayan leucogranites it is particularly rich in tourmaline. The granite is generally compositionally homogeneous although it is magmatically banded in both the upper and lower portions. The Bhagirathi leucogranite is situated structurally above the kyanite and sillimanite gneisses of the Vaikrita Group which, in turn, overlie the north-dipping Main Central Thrust zone of inverted metamorphic isograds. A pegmatite — aplite leucogranite sill and dyke swarm is present around the margins of the leucogranite. Vaikrita Group gneisses below the leucogranite contain a pronounced northeasterly component to generally randomly orientated mineral stretching lineations. This reflects localized reorientation of early, coaxially constrained, mineral growth by later, non-coaxial deformation. Various shear criteria in the gneisses immediately below the granite document the existence of a major zone of ductile NNE-SSW-directed extension across a northeastward dipping, low-angle normal fault zone. The top of the 1500–1800 m thick leucogranite sheet exposed on the peaks of Bhagirathi, Shivling and Thalay Sagar are intrusive into Martoli Formation metasediments of the Tethyan sequence which locally contain andalusite and staurolite. The roof complex shows numerous stoped blocks, xenoliths and elongate rafts of host rock in the top 100 m together with an extensive zone of layer-parallel leucogranite veining. The northward-dipping Bhagirathi leucogranite was intruded syn-tectonically during the ductile to brittle transition and has been deformed into linked, en echelon bodies within the extensional shear zone at the interface between the Vaikrita Group gneisses and the Tethyan sedimentary cover. The long axes of the leucogranite lenses lie parallel to the y-z plane of the finite strain ellipsoid for this extensional duplex. During extension, sub-orthogonal dilatational forces exceeded sub-horizontal shear stresses thus facilitating the repeated emplacement of sheeted granite melt, a process analogous to low-angle tension gash development. The final emplacement level in the crust must have been ultimately controlled by the density contrast between melt and country rocks, the thermal blanketing of the Tethyan sedimentary cover, and the extensional stress field along the top of the High Himalayan slab.
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