ALKALINE ROCKS AND CARBONATITES OF AMBA-DONGAR AND ADJACENT AREAS, DECCAN-ALKALINE PROVINCE, GUJARAT, INDIA .2. COMPLEXLY ZONED CLINOPYROXENE PHENOCRYSTS

Clinopyroxene phenocrysts in six samples, representing the composition al range in the alkaline rocks of the region, range overall from alumi nian titanian diopside (mg = Mg/[Mg + Fe-t] = 92) to aegirine (mg = 17 ), but Ca-pyroxenes are dominant (approximate to 90% of 851 analyses), Ca-Na pyroxenes minor (approximate to 10%) and Na-pyroxenes rare (<<1 %). Pyroxenes in associated subalkaline rocks (dolerites and basalts) are lower-Ca augites of distinct trend. Al and Ti correlate positively in the alkaline rock pyroxenes, but negatively with Si, mirroring dec reasing CaTiAl2O6 and CaSiAl2O6 components in more felsic host-rocks. Although the most evolved pyroxenes in each host-rock show increasing na (Na/[Na + Ca]) and Zr and decreasing mg as host-rock mg decreases, the most primitive pyroxenes in each host-rock do not change, implying they are not at equilibrium. Over 40 paired logitudinal and lateral t raverses across 21 phenocrysts reveal very complex zoning, in which up to five growth zones can be recognized in one crystal, separated by e ither sharp contacts or gradual transitions. These individual zones ma p shown one of seven zoning trends: normal (na up arrow as mg down arr ow moving outwards towards the rim), reverse (na down arrow as mg up a rrow), inverse (na up arrow as mg up arrow), converse (na down arrow a s mg down arrow), unzoned (no change), symmetric (na and mg display co mplementary boolean AND and boolean OR trends), or oscillatory (superi mposed on normal, reverse or unzoned). Ten distinct crystal types can also be recognized, showing particular sequences of zoning trends: for example, Type 1 shows a green, relatively sodic core surrounded by a more primitive rim or overgrowth, and has similar longitudinal and lat eral zoning; whereas Type 6 shows extremely erratic variations which d iffer longitudinally and laterally. Up to four of these types were fou nd in one host-rock, with little pattern in their distribution between different host-rocks. These pyroxenes are believed to record an intri cate history of stop-start differentiation, complicated by magma-mixin g, entry or disappearance of coprecipitating phases, and inheritance o f high-pressure precipitates. Apparently random superimposed variation s of Al, Ti and Si within some individual growth zones most probably r eflect kinetic (disequilibrium) effects, due to rapid growth and/or su percooling.