Collision of the North China and Yangtse Blocks and formation of coesite-bearing eclogites: Timing and processes☆

Aluminum in hornblende; an empirical igneous geobarometer

Late Archaean (2550–2520 Ma) juvenile magmatism in the Eastern Dharwar craton, southern India: constraints from geochronology, Nd–Sr isotopes and whole rock geochemistry

Calcic amphiboles were synthesized from a natural mid-ocean ridge basalt (MORB) in 39 experiments representing 24 sets of pressure-temperature (P-T) conditions ranging from 650‐950 8C, 0.8‐2.2 GPa, at ƒ controlled by the fayalite-magnetite-quartz (FMQ) buffer, O2 and Paqueous fluid 5 Ptotal. Experiments lasted up to 1630 h at low temperatures; in all cases, synthesized hornblendes were coarse-grained (5‐7 3 10‐15 mm) and chemically homogeneous. Over the investigated pressure range, Ca-amphibole coexisting with phases rich in Al and Ti gradually changes composition from sodic-calcic, Si-rich at low temperatures to calcic, Si-poor at high temperatures: it is barroisite at 650 8C, edenite at 700 8C, and pargasite at 800‐950 8C. Electron microprobe data were combined with 41 comparable analyses from the literature for Ca-amphiboles synthesized from MORBs at intermediate ƒ in order to erect a petrogenetic grid for the experimental range 0.0‐2.2 GPa, 450‐1050 O2 8C. Isopleths for Al2O3 in Ca-amphibole exhibit markedly negative P-T slopes, indicating increasing Al2O3 contents with both P and T. In contrast, TiO2 isopleths are nearly independent of P, demonstrating that TiO2 in Ca-amphibole increases almost exclusively as a function of T. For natural metabasaltic assemblages that contain coexisting Al-rich and Tirich phases, and closely approached chemical equilibrium under crustal or uppermost mantle conditions, this semiquantitative petrogenetic grid allows the simultaneous assignment of attendant P and T employing Ca-amphibole Al2O3 and TiO2 contents. However, during slow cooling, natural Ca-amphiboles may exsolve TiO2 as rutile, titanite, and/or ilmenite, but in general do not redistribute Al 2O3, so this thermobarometer must be applied with caution to inhomogeneous specimens.

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Experimental phase-equilibrium study of al- and ti-contents of calcic amphibole in morb : a semiquantitative thermobarometer

Both the coarse- and fine-grained varieties of the partly coesite-bearing pyrope-quartzites, their interlayered jadeite-kyanite rocks, and the biotite-phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents, which were also studied analytically by microprobe. Prior to pyrope and coesite growth, the Mg-rich metapelites were talc-kyanite-chlorite-rutile-ellenbergerite schists which — upon continued prograde metamorphism — developed first pyrope megacrysts in silica-deficient local environments at the expense of chlorite + talc + kyanite, and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc + kyanite. Based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations, a prograde PT-path — simplified for water activity = 1 — is constructed which passes through the approximate PT-conditions 16 kbar and 560° C, 29 kbar and 720° C, and finally up to 37 kbar at about 800° C, where the Mg-rich metapelite was a pyrope-coesite rock with phengite, kyanite, and talc still present. During the retrograde path, pyrope was altered metasomatically either into phlogopite + kyanite + quartz or, at a later stage, to chlorite + muscovite + quartz. Both assemblages yield PT-constraints, the latter about 7–9 kbar, 500–600° C. The country rock gneisses have also endured high-pressures of at least 15 kbar, but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies (about 5 kbar, 450° C). Microprobe data are presented for the following minerals: pyrope, ellenbergerite, dumortierite (unusually MgTi-rich), jadeite, vermiculite (formed after Na-phlogopite?), paragonite, and for several generations of phengite, chlorite, talc, phlogopite, dravite, and glaucophane in the high-pressure rocks, as well as for biotite, chlorite, phengites, epidote, garnet, albite, and K-feldspar in the country rock gneisses. An outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their PT-stability limits.

The pyrope-coesite rocks and their country rocks at Parigi, Dora Maira Massif, Western Alps ; Detailed petrography, mineral chemistry and PT-path

Chemical analyses of hornblendes from different regional metamorphic terrains and from rocks of different metamorphic grade have been compared. Hornblendes of lowpressure type are distinguished from hornblendes of high-pressure type by their AlVI and Si contents. The Ti content of hornblende is shown to increase with metamorphic temperature from the greenschist-amphibolite transition facies to the hornblende-granulite facies.

Al and Ti contents of hornblende, indicators of pressure and temperature of regional metamorphism

Two-feldspar geothermometry is applied to granulite-facies metapelites from the Highland Complex of Sri Lanka, for which very high peak temperatures of metamorphism have been inferred. Two-feldspar thermometry can be applied, even where only one feldspar has preserved its original bulk-composition in terms of Al-Si content, in spite of or because of unmixing to perthite or antiperthite. Different methods of integration of the unmixed feldspars to get the original bulk-composition are evaluated. Several types of feldspar assemblages are distinguished: (1) disequilibrium and near-equilibrium perthite - plagioclase pairs (several generations), (2) disequilibrium antiperthite - perthite pairs, (3) single mesoperthite, (4) near-equilibrium antiperthite-mesoperthite pairs. The highest temperatures, above 900 degrees C, are derived from antiperthite porphyroclasts with evenly spaced exsolution lamellae and rods, from antiperthite-mesoperthite pairs, and from perthite inclusions in garnet porphyroclasts. The causes for the preservation of ternary feldspar compositions are discussed. Feldspar recrystallization in response to strong deformation is related to thrusting of the Highland Complex onto the Vijayan Complex. It occurred at about 830-900 degrees C. In the main part of the Highland Complex, the feldspars recrystallized at ca. 680-760 degrees C. Mylonitization along the Digana shear zone took place at about 710 degrees C. Late retrograde recrystallization or growth of feldspar rims occurred in the range 460-590 degrees C. Two-feldspar geothermometry combined with published geochronological data reveal new insights in the metamorphic evolution of the Highland Complex of Sri Lanka.

Feldspar thermometry; a valuable tool for deciphering the thermal history of granulite-facies rocks, as illustrated with metapelites from Sri Lanka

The Archaean craton of southern India is characterized by a highly complicated and not yet fully understood geological history comprizing several cycles of sedimentation and volcanism, deformation and metamorphism in the span between about 3400 and 2500 m. y. The large scale regional variation in metamorphic grade observed today is essentially related to a metamorphic event at about 2600 m. y. ago which affected an older migmatite, gneiss-greenstone terrain (2900–3400 m. y.).

The Archean Craton of Southern India: Metamorphic evolution and P-T conditions

A glass of composition NaAlSi3O8 with small contents of Fe has been crystallized in the presence of water vapor under 1000 bars, at temperatures in the range of 250 to 700° C for varying periods of time. The synthetic crystals were investigated microscopically and by x-ray powder diffraction methods. Most of the crystals are twinned according to the albite law, but other twins are found too. Size and form of the crystals seem to depend on the temperature of crystallization. The optic axial angles (2V

 x
 ) of the albites being measurable on the universal stage are in the range of 50 to 100 degrees. There seems to exist a linear relation between the 2ϑ131−2ϑ1¯31 values (used as an indicator for the degree of ordering) and the 2V

 x
 values. The change of the lattice parameters from albite (D) to albite (O) is suggested to deviate slightly from linearity. In contrast to previous investigations the steady value of 2ϑ131−2ϑ1¯31 which is characteristic of the temperature of crystallization was reached more rapidly. These results suggest that small content of Fe in the glass had a catalytic effect on the Al/Si ordering.

Über die Synthese von Albiten bei Temperaturen von 250 bis 700° C

The Precambrian crust of northern Finland has been the subject of detailed geochemical and petrological studies. The geochemical data and the present-day geotectonic setting suggest the existence of an old island arc system with the West Inari schist zone (Tana River belt) as an outer tholeiitic arc, the granulite complex as a calcalkaline inner arc, and a stable continental block (granite gneiss complex + Kola nucleus) in the northeast.

Peter Raase

Papers

Al and Ti contents of hornblende, indicators of pressure and temperature of regional metamorphism

Feldspar thermometry; a valuable tool for deciphering the thermal history of granulite-facies rocks, as illustrated with metapelites from Sri Lanka

The Archean Craton of Southern India: Metamorphic evolution and P-T conditions

Über die Synthese von Albiten bei Temperaturen von 250 bis 700° C

The Precambrian of Finnish Lapland: Evolution and regime of metamorphism