Mineralogy and metamorphic history of norite dykes within granulite facies gneisses from Sand Mata, Rajasthan, NW India RAM S. SHARMA, 1 JANE D. SILLS 2 AND M. JOSHI 1 1 Department of Geology, Banaras Hindu University, Varanasi 22100, India. 2 Department of Geology, Leicester University, Leicester LE1 7RH, England. Abstract Metanorite dykes intrude the Banded Gneiss Complex at various places in Rajasthan, N.W. India. They show neither chilled margins nor gradational contacts with the country rock amphibolite or granulite facies gneisses. They have ophitic to subophitic texture with strongly zoned subcalcic clinopyroxene and orthopyroxene, olivine and plagioclase, with subsidiary biotite. During slow cooling a series of reaction coronas developed with garnet forming round biotite, ilmenite and orthopyroxene; hornblende round pyroxenes and orthopyroxene, hornblende + spinel round olivine, which may be totally replaced. It is inferred that the dykes crystallised from a tholeiitic magma at about 1100-1150 ~ and were intruded during the waning stages of granulite facies metamorphism. The corona minerals grew at about 650- 700 ~ A series of reactions to account for the development of the coronas is proposed using measured mineral compositions. Although these reactions do not balance for individual corona formation, metamorphism was probably isochemical with Ca, Na, K, Ti, Si and H20 only mobile on the scale of a thin section. Si and H20 were possibly mobile on a larger scale. KEYWORDS" norites, dykes, gneisses, granulite facies, Rajasthan, India. Introduction METANORITE dykes of uncertain age intrude the upper amphibolite to granulite facies rocks of the Banded Gneiss Complex (BGC) at various places in Rajasthan, NW India (Fig. 1; Gupta, 1934; Heron, 1953). Where the dykes intruded granulite facies gneisses they have partially re-equilibrated to granulite fades conditions and where they cut amphibolite facies gneisses they are hornblende- rich. The BGC is older than 2.58 Ga (Crawford, 1970) and possibly as old as 3.5 Ga (McDougall et al., 1983). At Sand Mata, olivine-bearing, unfoliated meta- norite dykes crosscut the high-pressure granulites and interlayered garnet amphibolites and meta- pelites of the BGC (Sharma et al., in prep.). The dykes are 10-15 m wide and show neither chilled margins nor gradational contacts with the country rock. They are distinct in texture and mineral assemblage from the garnet amphibolites and granulites. They have ophitic to subophitic texture, but at Sand Mata also contain garnet, recrystallised Mineralogical Magazine, June 1987, Vol. 51, pp. 207-15 (~ Copyright the Mineralogical Society amphibole and subordinate biotite which indicate that the basic intrusions have been partially metamorphosed. The gneisses from Sand Mata generally trend NW and have suffered three phases of deformation. The earliest recognisable phase (F1) comprises rootless folds. The second deformation (F2) pro- duced N-NW-trending isoclinal or reclined folds whose axes parallel the regional folds. It is thought that F 2 was coeval with the upper amphibolite to granulite facies metamorphism related to the Aravalli Mountain Orogeny (1.6-0.94 Ga). F 3 de- formation is heterogeneous but trends NE-SW and probably occurred during the waning stages of the main metamorphism (Sharma et al., in prep.). The metanorite dykes trend SW and cut the regional foliation, indicating emplacement prior to or during F 3. The aim of this paper is to describe the sequence of textural and compositional changes occurring in the dykes and to determine to the conditions of metamorphism.