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2 2012

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New evidence for high-pressure metamorphic rocks in Western Svalbard

Kośmińska K., Majka J., Krumbholz M., Klonowska I., Manecki M., Czerny J.

Geology, Geophysics and Environment

2012

Vol. 38, no. 4

488--489

During a field expeditions to Nordenskiold Land and Wedel Jarlsberg Land (western Svalbard), previously unrecognised high-grade metamorphic rocks were observed. These rocks are represented by blueschists sensu stricto and blueschist facies metapelites. In Nordeskiold Land, blueschists occur in form of isolated bodies/tectonic lenses of different size enclosed within metasediments. The rocks in question are dark green, consist mainly of reddish garnet porphyroblasts and greenish and bluish amphiboles (including ferroglaucophane). They also contain chlorite, epidote, phengite, biotite, pumpellyite, quartz and albite. Garnet forms euhedral to subhedral poikiloblasts containing voluminous inclusions of epidote, albite, quartz, amphibole and titanite. Garnet shows chemical compositional variation from Alm49Pyr1Grs33Spss17 in the cores to Alm63Pyr2Grs32Spss3 in the rims. Gradual changes in chemical zoning as well as bell-shaped Spss content profiles suggest one-step, progressive garnet growth. P-T estimates based on thermodynamic modeling (using Perple_X'O7) in the NCKFMMnASHT system suggest peak pressure conditions of ca. 20 kbar and 480°C. P-T estimates are based on garnet and phengite compositional isopleths and stability field of the paragenetic assemblage (Chl-Ph-Ampl-Amp2-Grt-Spn). Tectonically, the whole area is characterized by a moderate, 50°-dipping of SO and SI (the main metamorphic foliation) to the north. Deviations from this direction are small and occur only locally. They are mostly caused by the presence of rigid metamafic bodies, around which the common orientation of the bedding and the metamorphic foliation is disturbed. The observed stretching lineations plunge towards the N-NNE and are therefore in very good agreement with the orientation of SO and SI. Moreover, shear sense indicators such as commonly observed sigma elasts clearly indicate a transport direction from the north to the south. The structural observations are in good agreement with those collected south of the Nordenskiold Land, in the northern part of Wedel Jarlsberg Land (Antoniabreen area), where high grade augen gneisses and metapelites thrusted onto typical low-grade rocks of SW Svalbard occur. The metapelites contain mainly garnet, muscovite, biotite, chlorite and quartz. Most of the garnet porphyroblasts show two distinct growth zones. Garnet-I (Alm64Pyr6Grs9Spss21) represents the inner growth zone, forming inclusion-rich garnet core and garnet-II (Alm52Pyr2Grs30Spss16) builds the outer growth zone, forming euhedral garnet rims. Some of the smaller garnet porphyroblasts show only a single growth zone (representing grt-II). Preliminary P-T estimates based on thermodynamic modeling in the NCKFMMnASHT system indicate the growth of garnet-I at ca. 550°C and relatively low pressure (ca. 5 kbar), whereas garnet-II grew at ca. 500°C and 12 kbar. The P-T estimates for garnet-I are based on garnet, biotite and plagioclase compositional isopleths and stability field of the paragenetic assemblage (Bt-Chl-Pl-Ms-Grt), whereas for garnet-II on the garnet and muscovite isopleths and stability field of the assemblage (Bio-Chl-Ms-Grt-Ab). We suggest that the growth of garnet-I is connected to the Late Neoproterozoic Torellian event, while garnet-II has been growing under blueschist facies conditions, hence most probably during the Caledonian orogeny. Tectonic observations and P-T conditions are similar in both studied areas. It suggest that vast parts of the Caledonian basement of Nordeskiold Land and northern Wedel Jarlsberg Land were metamorphosed under high pressure conditions. These blueschist facies rocks may be an equivalent of the high pressure unit known from Oskar II Land (Motalafjella region), occurring to the north from the research area. Further, detailed petrological and geochronological studies to verify this hypothesis are in progress.

Metavolcanics of Nordenskiold Land from SW Svalbard as an example of New Ocean Crust

Gołuchowska K., Barker A. K., Manecki M., Majka J., Czerny J.

Geology, Geophysics and Environment

2012

Vol. 38, no. 4

473

This study concerns late Neoproterozoic metavolcanics from southwestern Svalbard in an investigation to trace the evolution of oceans plate rifting. A number of outcrops of these rocks are connected with the regional Torellian unconformity, which is associated with a Late Neoproterozoic orogenesis event - post 640 Ma. This unconformity divides the younger metasediments of the Sofiebogen Group from older metasedimentary sequence of the Deilegga Group. Samples have been collected from Nordenskiold Land belonging to the Sofiebogen Group, which is situated between Bellsund and Isfjorden, to the north of Wedel Jarlsberg Land. Field observation reveals that these metavolcanics very often occur as a pillow lavas and lavas. They contain mineral assemblages typical for greenschist facies metamorphism such as: actinolite, chlorite, epidote, albite, but some of them contain garnets and glaucophane, which is typical for blueschist facies conditions. Based on a total alkali silica diagram they are classified as tholeitic basalts. Spider diagram shows patterns similar to MORB, where the REE are flat. The LILE except Sr are depleted, but this depletion is caused probably by metamorphism. However, the metavolcanics of Nordenskiold Land are also relative depleted in HFSE (Th/Yb = 0.2-0.31 and Nb/Yb = 2.9-4.4) and LREE (Lan/Smn = 1.0-1.5). Trace elements like Nd, Th, Yb and La show trends that could be associated with fractional crystallization. The geochemical character of metavolcanics from Nordenskiold Land, as well as, their forms (pillow lavas) suggests that they created beneath the sea level, as new ocean crust.