140 SHORT COMMUNICATIONS to a structure comprising hundreds of thousands in the gel could not be incorporated in the of individual sub-millimetre crystals of quartz by crystalline quartz and would be pushed ahead of any mechanism of growth from a liquid. It is the growing crystals (Sweetman, 1988). This therefore suggested that the plates developed accounts for the concentration of hematite on the along fractures which formed during the de- surface of quartz crystals lining the vugs and along hydration of a silica gel according to the following the line where two rows of orthogonal crystals mechanism. meet. The central position of the contact of the Following its injection into a cavity in the two sets of quartz crystals from adjacent central country rock, the silica gel would begin to cool, zones indicates simultaneous equal growth away dehydrate and subsequently contract. In a similar from the two central zones, and the cross-cutting way to the formation of columnar joint patterns in relationships indicate that all the central zones basaltic lava flows, the contraction would result in formed prior to the growth of the orthogonal individual 'cells' which would contract as separate quartz. The overall reduction in volume resulting units. Within each of these cells fractures would from the crystallisation of the gel would account have developed in response to the contraction. It for the abundance of the vugs, and the average is interesting that all the plates which subse- density of the whole rock (2.48 g cm -3) may give quently developed along these fractures are an indication of the anhydrous density of the perfectly planar with no evidence of curvature, so original gel. the fracturing behaviour of the gel must be different to that of other non-crystalline solid Acknowledgements. We wish to thank an anonymous materials such as glass in which conchoidal referee for very helpful comments on this paper. fractures would be expected. The fractures would have provided pathways for the ingress of References aqueous solutions (perhaps containing hematite Donaldson, C. H. (1982) Spinifex textured komatiites: in suspension) and promoted the crystallisation of a review of textures, compositions and layering. In thin central zones of randomly orientated quartz Komatiites (Arndt, N. T. and Nisbet, E. G., eds.), crystals. Once a central zone was formed, the Allen and Unwin, London, 213-44. quartz would act as a nucleation centre for the Orpen, J. L., Swain, C. J., Nugent, C., and Zhou, P. P. recrystallisation of the remaining silica gel and (1989) Wrench-fault and half-graben tectonics in the quartz crystals grew orthogonally from the central development of the Palaeozoic Zambezi Karoo zone, their orientation being controlled by the Basins in Zimbabwe--the 'Lower Zambezi' and simultaneous growth of many crystals outwards 'Mid-Zambezi' basins respectively--and regional from the same plane. However as quartz has a implications. J. African Earth Sci., 8, 215-29. Sweetman, T. M. (1988) Fault breccias in the Leinster higher density than the silica gel, there would be Granite. Irish J. Earth Sci., 9, 125-32. further contraction towards the crystallisation centres, ultimately resulting in the development [Manuscript received 1 April 1990; of vugs. During the recrystallisation of the quartz revised 6 May 1990] between two central zones any hematite present Copyright the Mineralogical Society KEVWORDS: quartz, silica gel, Zimbabwe. Department of Geology, THORLEY M. SWEETMAN University of Zimbabwe, PAUL L. TROMP P. 0. Box MP 167, Mount Pleasant, Harare, Zimbabwe MINERALOGICAL MAGAZINE, MARCH 1991, VOL 55, PP. 140-142 Staurolite from a metabasite and its paragenesis THIS study deals with the gedrite-staurolite India. The rocks of the study area have been assemblage from a metabasite (amphibolite) in classified into an older Vinjamuru schist complex the Vinjamuru area (Lat 14050 , and Long 79035 ') and a younger Udayagiri Group (Vasudevan of the Nellore granite-greenstone terrain of S. et al., 1977). Metapelites (quartz-mica- SHORT COMMUNICATIONS 141 plagioclase-staurolite-garnet-kyanite schists), Table 1. Elect~n probe analyses of ataurolltes mid 8edrtte partly migmatised due to the injection of quartzo- feldspathic material, are the oldest rocks and Staurolite these are followed by quartzites, dolomites and Gedrite amphibolites. The Udayagiri Group is made up of RS 91 RS 112 metarhyolites, carbonate rocks, quartzites, meta- siq 28.70 27.65 44.54 andesites and metadacites. A NNW-SSE trend- rio, 00.49 00.46 00.13 AL~O~ 52.38 54.47 16,49 ing shear zone, proved to be a thrust by DSS ~o 13.28 12.94 19.71 studies (Kaila et al. 1979), traverses the study Mn0 00.14 00.13 00.29 ' MgO 03.06 02.56 15.56 area; it brought the rocks of Vinjamuru Group c,o 000.02 00.01 00.49 00.02 00.01 01.81 into juxtaposition with the Udayagiri Group KzONo~O 00.06 00.04 00.04 (Vasudevan et al., 1977). The rocks in the vicinity cr, o, 00.04 00.04 00.03 B,o 00.05 00.04 00.04 of the shear zone have been subjected to retro- Total 98.23 98.35 99.13 gression and the hornblende is partially replaced 44(0) 44(0) 22(0) by chlorite. 7.561 7.277 6.096 Amphibolites occur in schistose and massive AI(4)si 1.904 varieties and are generally composed of horn- A~6~ 16.299 16.900 0.759 ri 0.097 0.090 0.013 blende, plagioclase, orthoclase, opaques + r~ 2.931 2.849 2.256 garnet, and have an igneous parentage [Moeen M. 0.032 0.029 0.034 Mg 1.204 1.004 3.174 and Babu, 1988). They are intrusive bodies and in co 0.005 0.003 0.072 0.009 0.003 0.480 the well sections, located at SE of Vinjamuru KNo 0.016 0.013 0.003 Taluq office, it has been observed that the cr 0.010 0.009 0.003 0.005 0.005 0.002 amphibolites are intruded into the dolomite, Bo 0.291 0.260 XM9 resulting in contact metamorphism. The contact is r 8.000 characterised by the development of tremolite- c 5.000 B 1.788 diopside-dolomite rock. In other localities, A - 0.003 where amphibolites are associated with quart- zites, thin layers of amphibolites occur in quart- zites. Towards the west of Nandigunta (Lat 14050'45 " and Long 79~ an amphibolite essentially made up of gedrite is exposed. It runs for about 150 metres in a NW-SE direction and the contact with the hornblende schists is not exposed. In this area, the contact between the Table 2. XRF ~malysis of the host rocks hornblende schists and the overlying metarhyo- lites is well exposed. When traced towards the amphibolite-metarhyolite contact, the horn- RS 91 RS 112 blende schist becomes richer in biotite and poorer in hornblende, indicating an interaction between si~ 49.09 59.12 rhyolite lava and the amphibolite. The petrogra- Tie2 01.4O 01.20 phy provides evidence of the conversion of hornblende to biotite. A1203 12.90 17.62 The gedrite-bearing amphibolites are essen- Ve203 02.52 04.82 tially made up of gedrite plus staurolite, which is absent in other amphibolites, and minor amounts F~o 17.48 07.98 of biotite, garnet and occasionally plagioclase. M,,o 00.34 00.08 Hornblende is sparse in amount and few needles of sillimanite occur and they seem to be released Mgo 14.11 04.60 from gedrite, coo O0.45 0O. 40 Gedrite occurs as columnar aggregates with feeble pleochroism and parallel extinction. N~176 01.1o 01.50 Staurolite appears as subidioblastic grains co- K2o 00.50 03.14 existing with gedrite, biotite and garnet. Gedrite-staurolite and gedrite-biotite-staurolite P2~ 00.13 O0.07 are the most commonly observed assemblages. Staurolite also occurs as inclusions in garnet. Total 100.02 100.53 Inclusions of quartz, biotite and opaques are 142 SHORT COMMUNICATIONS common in staurolite and twinning is well devel- Further, the present mineral assemblage is in oped. Biotite occurs both as the product of contrast with the Hbl + P1 + Ky (or Cor) + St + prograde metamorphism and as an alteration Ru assemblage reported by Gibson (1978) in a product of gedrite. The former coexists with pure metabasite. The low XMg value of the gedrite, staurolite, biotite, and garnet, and also staurolite indicates that it has formed at usual as inclusions in them. Garnet occurs as well metamorphic conditions as Mg rich rocks yield developed porhyroblasts with sieve structure Mg-staurolites at high P and T conditions (Masaki carrying the inclusions of staurolite and biotite. and Qijia, 1988). Staurolite in the metapelite occurs as perfect idioblasts in association with micas, garnet, Acknowledgements. The author expresses thanks to plagioclase and quartz. Prof. V. R. R. M. Babu for his suggestions and to The staurolites are Fe-rich (Table 1). Since the UGC, New Delhi, for the financial assistance. Thanks metapelite (RS 112) is aluminous and siliceous are due to the Director, NGR1, Hyderabad, and concerning scientists for their help in carrying out the (Table 2) its composition is favourable for the XRF and EPMA studies. development of staurolite as mentioned by Grew and Sandiford (1984). In the case of metabasite (RS 91), the highly augmented FeO and MgO References contents favour the development of ferromag- nesium minerals. The impoverishment of Ca not Gibson, G. M. (1978) Mineral. Mag., 42, 153-4. only causes the development of gedrite at the Grew, E. S. and Sandiford, M. (1984) Contrib. Mineral. expense of hornblende but also limits the forma- Petrol., 87, 337-50. tion of plagioclase.