The composite airborne total intensity map of the Southern Granulite Terrain (SGT) at an average elevation of 7000' (≈ 2100 m) shows bands of bipolar regional magnetic anomalies parallel to the structural trends suggesting the distribution of mafic/ultramafic rocks that are controlled by regional structures/shear zones and thrusts in this region. The spectrum and the apparent susceptibility map computed from the observed airborne magnetic anomalies provide bands of high susceptibility zones in the upper crust associated with known shear zones/thrusts such as Transition Zone, Moyar-Bhavani and Palghat-Cauvery Shear Zones (MBSZ and PCSZ). The quantitative modelling of magnetic anomalies across Transition Zone, MBSZ and PCSZ suggest the presence of mafic rocks of susceptibility (1.5-4.0 × 10−3 CGS units) in upper crust from 8-10 km extending up to about 21-22 km, which may represent the level of Curie point geotherm as indicated by high upper mantle heat flow in this section.Two sets of paired gravity anomalies in SGT and their modelling with seismic constraints suggest gravity highs and lows to be caused by high density mafic rocks along Transition Zone and Cauvery Shear Zone (CSZ) in the upper crust at depth of 6-8 km and crustal thickening of 45-46 km south of them, respectively. High susceptibility and high density rocks (2.8 g/cm3) along these shear zones supported by high velocity, high conductivity and tectonic settings suggest lower crustal mafic/ultramafic granulite rocks thrusted along them. These signatures with lower crustal rocks of metamorphic ages of 2.6-2.5 Ga north of PCSZ and Neoproterozoic period (0.6-0.5 Ga) south of it suggest that the SGT represents mosaic of accreted crust due to compression and thrusting. These observations along with N-verging thrusts and dipping reflectors from Dharwar Craton to SGT suggest two stages of N-S directed compression: (i) between Dharwar Craton and northern block of SGT during 2.6-2.5 Ga with Transition Zone and Moyar Shear towards the west as thrust, and (ii) between northern and southern blocks of SGT with CSZ as collision zone and PCSZ as thrust during Neoproterozoic period (0.6-0.5 Ga). The latter event may even represent just a compressive phase without any collision related to Pan-African event. The proposed sutures in both these cases separate gravity highs and lows of paired gravity anomalies towards north and south, respectively. The magnetic anomalies and causative sources related to Moyar Shear, MBSZ and PCSZ join with those due to Transition Zone, Mettur and Gangavalli Shears in their eastern parts, respectively to form an arcuate-shaped diffused collision zone during 2.6-2.5 Ga.Most of the Proterozoic collision zones are highlands/plateaus but the CSZ also known as the Palghat Gap represents a low lying strip of 80-100 km width, which however, appears to be related to recent tectonic activities as indicated by high upper mantle heat flow and thin crust in this section. It is supported by low density, low velocity and high conductive layer under CSZ and seismic activity in this region as observed in case of passive rift valleys. They may be caused by asthenospheric upwarping along pre-existing faults/thrusts (MBSZ and PCSZ) due to plate tectonic forces after the collision of Indian and Eurasian plates since Miocene time. 相似文献
Contact metamorphism in the aureole of the 1322 Ma Makhavinekh Lake Pluton, northern Labrador, affected monazite and zircon in the adjacent 1850 Ma metapelitic gneisses. Transformation of regional garnet and sillimanite to lower-pressure symplectitic intergrowths of cordierite, orthopyroxene, and spinel was accompanied by resorption of inherited monazite inclusions in garnet coupled with the appearance of coronitic high-Y monazite rims. In situ ion-microprobe dating is used to show that high-Y rims formed during contact metamorphism. Liberation of Y and HREE from garnet also gave rise to new xenotime growth. The coronitic nature of monazite overgrowths reflects the diffusion-controlled nature of net-transfer reactions whereas its higher Y composition reflects equilibration with xenotime at peak T (> 800 °C) conditions in the inner aureole. Very thin overgrowths on inherited zircon were also encountered, but only where zircon is surrounded by the symplectitic assemblage, reflecting liberation of Zr from garnet. Although these overgrowths are too thin to date using conventional ion-microprobe techniques, well-developed triple junctions between zircon and orthopyroxene suggests that they grew in textural equilibrium with the contact metamorphic assemblage.
In contrast to monazite, inherited zircon remained intact during contact metamorphism, exhibiting no change in morphology (other than the growth of thin rims) or internal zoning throughout the aureole. However, inherited sector-zoned zircons of anatectic origin display evidence for intracrystalline Pb redistribution in the inner aureole. In these samples, ion-microprobe analyses encountered heterogeneous Pb signals and a dispersion of 207Pb / 206Pb dates away from the well constrained 1850 Ma age of regional metamorphism. Whereas analyses from the outer aureole faithfully record the age of regional metamorphism, those from the inner aureole are normally and reversely discordant and distributed along a line collinear with a 1850 to 1322 Ma discordia. This disturbance is correlated with proximity to the pluton implying that Pb was mobile in the zircon lattice during contact metamorphism. Most grains are characterized by apparent Pb loss from low-U domains and apparent Pb gain in higher-U domains. These data are interpreted to reflect recovery of strained crystalline domains leading to expulsion of Pb* that was able to efficiently diffuse into higher-U domains that were partly amorphous prior to rapid reheating in the inner aureole. 相似文献
Here new mineralogical data is presented on the occurrence of K-feldspar in granulite-facies metagabbronorite xenoliths found in recent alkaline lavas from Western Sardinia, Italy. The xenoliths originated from the underplating of variably evolved subduction-related basaltic liquids, which underwent cooling and recrystallisation in the deep crust (T = 850–900 °C, P = 0.8–1.0 GPa). They consist of orthopyroxene + clinopyroxene + plagioclase porphyroclasts (An = 50–66 mol%) in a granoblastic, recrystallised, quartz-free matrix composed of pyroxene + plagioclase (An = 56–72 mol%) + Fe–Ti oxides ± K-feldspar ± biotite ± fluorapatite ± Ti-biotite. Texturally, the K-feldspar occurs in a variety of different modes. These include: (1) rods, blebs, and irregular patches in a random scattering of plagioclase grains in the form of antiperthite; (2) micro-veins along plagioclase–plagioclase and plagioclase–pyroxene grain rims; (3) myrmekite-like intergrowths with Ca-rich plagioclase along plagioclase–plagioclase grain boundaries; and (4) discrete anhedral grains (sometimes microperthitic). The composition of each type of K-feldspar is characterized by relatively high albite contents (16–33 mol%). An increasing anorthite content in the plagioclase towards the contact with the K-feldspar micro-vein and myrmekite-like intergrowths into the K-feldspar along the plagioclase–K-feldspar grain boundary are also observed. Small amounts of biotite (TiO2 = 4.7–6.5 wt.%; F = 0.24–1.19 wt.%; Cl = 0.04–0.20 wt.%) in textural equilibrium with the granulite-facies assemblage is present in both K-feldspar-bearing and K-feldspar-free xenoliths. These K-feldspar textures suggest a likely metasomatic origin due to solid-state infiltration of KCl-rich fluids/melts. The presence of such fluids is supported by the fluorapatite in these xenoliths, which is enriched in Cl (Cl = 6–50% of the total F + Cl + OH). These lines of evidence suggest that formation of K-feldspar in the mafic xenoliths reflects metasomatic processes, requiring an external K-rich fluid source, which operated in the lower crust during and after in-situ high-T recrystallisation of relatively dry rocks. 相似文献