Acicular Hornblende Schists and Associated Metabasic Rocks from North-West Pakistan

D. R. C. Kempe
Department of Mineralogy, British Museum (Natural History), London SW7 5BD

Synopses: A suite of unusual acicular hornblende schists is found at Warsak (34° 10′ N. 71°C 23′ E.), some 30 km WNW. of Peshawar, North-West Pakistan. The schists occur in association with sill-like masses of amphibolite facies meta-igneous rocks, possibly a metamorphosed gabbroic and doleritic, dioritic, and granitic calc-alkaline series, and intrusions of alkaline granite and microgranite. These rocks lie within Palaeozoic metasediments and all are folded synclinally. The area was first described by Ahmad et al. (1969).

The rocks of the acicular hornblende schist suite (fig. I) consist essentially of large, idioblastic hornblende crystals—comprising some 30–60 % of the rock—in a fine-grained granoblastic matrix of oligoclase feldspar with minor quartz, iron oxide, biotite, and rutile. The boat-shaped, blue-green hornblendes, twinned on {100}, reach 5 mm in length and are often grouped in sprays or clusters; they are sieved with quartz, ilmenite, and patchy calcite. The groundmass also contains patches and veins of calcite, associated with ragged patchy areas of reddish brown, amorphous iron oxide or hydroxide, perhaps introduced hydrothermally. The schists coarsen in grain size outwards through the sill-form, the hornblendes reaching 4 cm, become rich in chlorite and biotite, and grade into the apparently metagabbroic amphibolitic rocks.

The acicular hornblende schists have a generally basaltic composition, while the hornblendes are tschermakitic. An analysed amphibole has 16·5% Al2O3, is slightly zoned with alumina increasing from core to rim, and has the formula: K0·06Na0·50Ca1·90Mg2·13Fe1·492+Mn0·02Fe3+0·14Ti0·04Alvi1·16Si6·31Aliv1·69O21·97(OH,F)2·03

Three hypotheses of the origin of the rocks were considered; metamorphism of a suite of basic tufts is the most favoured. Origins involving metamorphism of a lamprophyre-appinite suite or of a gabbroic marginal facies are therefore rejected. Texturally the rocks strongly resemble metatuffs (fig. 2) that occur associated with metagreywackes in the Otago schists of New Zealand (Turner, 1933); like many similar Alpine schists they also contain tschermakitic hornblendes. The Warsak tufts could have formed originally, together with the meta-igneous rocks, possibly as lavas, in an inter-plate tectonic environment; the high A1 content of the hornblende, which is typical of many amphiboles from the surrounding alpine environment in North-West Pakistan, and other mineralogical evidence in the region support a highpressure environment for the metamorphism of the rocks to just within the amphibolite facies, at moderate temperatures of approximately 465 °C.

Mineralogical Magazine; September 1978 v. 42; no. 323; p. 405-406; DOI: 10.1180/minmag.1978.042.323.13
© 1978, The Mineralogical Society
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