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High-pressure and ultrahigh-pressure (UHP) eclogite-bearing metamorphic assemblages in the North Tianshan of Kyrgyzstan are known from the Aktyuz and Makbal areas, where eclogites and garnet amphibolites are associated with continental rocks such as granitoid gneisses in Aktyuz and shallow-water clastic (passive margin?) metasediments in Makbal. We present the first Lu-Hf isotope data for an eclogite and two garnet amphibolite samples from the two metamorphic terranes which, combined with petrological analysis, tightly constrain the age of high-pressure metamorphism in the Kyrgyz North Tianshan. A fivepoint isochron for an Aktyuz eclogite sample provides a LuHf age of 474.3 ± 2.2 Ma, and a four-point isochron on a Makbal sample corresponds to 470.1 ± 2.5 Ma. A prograde, subduction-related path is inferred for both samples with peak P-T conditions ranging from 1.4 to 1.6 GPa and 610-620°C. A further Makbal sample provided a significantly older Lu-Hf age of 486 ± 5.4 Ma, most likely due to late alteration in the sample (late addition of unradiogenic Hf). We conclude that garnet growth in all three samples occurred around ca. 474 Ma and that these rocks likely experienced UHP metamorphism contemporaneously. Our results support previous geochronological evidence for an Early Ordovician collision belt in the North Tianshan and allow refinement of a tectonic model involving subduction of thinned continental crust to considerable depth along the margin of a small microcontinent. Keywords Lu-Hf * Mafic dike * Aktyuz * Makbal * Northern Tianshan * Kyrgyzstan, Introduction The Tianshan orogen of north-western China, Kyrgyzstan, Kazakhstan and Uzbekistan is part of the Central Asian Orogenic Belt (CAOB; Fig. 1) which evolved over some 800 Ma from the [...]

Byline: Antonio Garcia-Casco (1), A. Rodriguez Vega (2), J. Cardenas Parraga (3), M. A. Iturralde-Vinent (4), C. Lazaro (3), I. Blanco Quintero (3), Y. Rojas Agramonte (5,2), A. Kroner (5), K. Nunez Cambra (6), G. Millan (6), R. L. Torres-Roldan (3), S. Carrasquilla (3) Keywords: Jadeitite jade; High pressure; Fluids; Subduction; Caribbean; Eastern Cuba Abstract: A new jadeitite jade locality has been discovered in the serpentinite-matrix subduction melange of the Sierra del Convento (eastern Cuba) in a context associated with tectonic blocks of garnet-epidote amphibolite, tonalitic--trondhjemitic epidote gneiss, and blueschist. The mineral assemblages of jadeitite jade and jadeite rocks are varied and include combinations of jadeite, omphacite, albite, paragonite, analcime, clinozoisite-epidote, apatite, phlogopite, phengite, chlorite, glaucophane, titanite, rutile, zircon, and quartz formed during various stages in their P--T evolution. Field relationships are obscure, but some samples made almost exclusively of jadeite show evidence of crystallization from fluid in veins. In one of these samples studied in detail jadeite shows complex textural and chemical characteristics (including oscillatory zoning) that denote growth in a changing chemical medium. It is proposed that interaction of an Al--Na rich fluid with ultramafic rocks produced Al--Na--Mg--Ca fluids of varying composition. Episodic infiltration of these fluids, as a result of episodic opening of the veins, developed oscillatory zoning by direct precipitation from fluid and after reaction of fluid with pre-existing jadeite. The latest infiltrating fluids were richer in Mg--Ca, favouring the formation of omphacite and Mg--Ca rich jadeite in open voids and the replacement of earlier jadeite by fine-grained omphacite + jadeite at 550--560degC. This new occurrence of jadeite in Cuba opens important perspectives for archeological studies of pre-Columbian jade artifacts in the Caribbean region. Author Affiliation: (1) Departamento de Mineralogia y Petrologia, Universidad de Granada, and Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad de Granada, Avda Fuentenueva sn, 18002, Granada, Spain (2) Departamento de Geologia, Instituto Superior Minero-Metalurgico, Las Coloradas s/n., Moa, 83329, Holguin, Cuba (3) Departamento de Mineralogia y Petrologia, Universidad de Granada, Avda Fuentenueva sn, 18002, Granada, Spain (4) Museo Nacional de Historia Natural, Obispo no. 61, Plaza de Armas, 10100, La Habana, Cuba (5) Institut fur Geowissenschaften, Universitat Mainz, 55099, Mainz, Germany (6) Instituto de Geologia y Paleontologia, Via Blanca y Carretera Central, San Miguel del Padron, 11000, Ciudad Habana, Cuba Article History: Registration Date: 24/11/2008 Received Date: 19/07/2008 Accepted Date: 24/11/2008 Online Date: 12/12/2008 Article note: Communicated by H. Keppler.

In the Sierra del Convento melange peraluminous trondhjemitic-tonalitic rocks, formed by partial melting of subducted oceanic crust at ca. 15 kbar, 700[degrees]C, contain millimeter-sized crystals of paragonite that appear to be of magmatic origin. These crystals are replaced by plagioclase and quartz along the rims. Chemically relict paragonite cores have high K contents (up to 0.35 apfu), whereas K is low at the rims (down to ca. 0.10 apfu). Calcium is relatively high in the cores and rims (0.06-0.11 apfu). The relict core compositions contrast significantly with small matrix paragonite crystals, which formed during subsequent subsolidus retrogression ofrelictic magmatic plagioclase at high pressure/low temperature (ca. 400[degrees]C, 8 kbar). Textural relations, mineral composition, and calculations of phase relationships in the system NCASH strongly suggest that primary paragonite formed by crystallization of a silicate melt. This appears to be the first reported occurrence of magmatic paragonite. Keywords: Magmatic paragonite, trondhjemite, melange, subduction

A study was conducted on a rare type of phengitic muscovite decomposition and breakdown in high P-T leucogneisses from the Betic alpine belt, Spain. The complex reaction microtextures and heterogeneities found in these specimens were formed due to a tectonic setting which induced rapid near-isothermal decompression in the original high P-T assemblages at high temperature. TEM images showed that biotite intergrowths reflect various frozen growth stages and that deformation defects in the muscovite affected biotite-quartz nucleation.

Several geochronological studies of the detrital zircon Jurassic sandstones of western Cuba (San Gayetano Formation) are conducted to explain the Jurassic Paleogeography of the NW Proto-Caribbean. The results demonstrate that the sediments of the San Gayetano Formation were a part of the supercontinent Pangea in pre mid-Oxfordian time.

Research was conducted to examine the origin of cordierite in mineral assemblages consisting of garnet, staurolite, cordierite, biotite, kyanite, sillimanite, andalusite, muscovite and quartz. Also, its bearing on current P-T grids and the development of extensional collapse of the Beltic-Rif Belt was analyzed. Results are in agreement with independent evidence suggesting that the termination of alpine metamorphism in the western Betic-Rif Belt was associated with the extensional collapse of thickened crust.