Quartz textures in dioritic rocks of hybrid origin

Abstract

Quartz in diorites and tonalites characteristically occurs in three textural forms a) Interstitial to feldspars (and other minerals) This is the dominant relationship, and poses the problem of the crystallization sequence involving these two minerals Fractional crystallization does not allow the produetion of a residual liquid of SiOz composition Some feldspars must therefore have crystallized with the quartz - in what proportions and with what composition must be determined - which means that this sort of geometry, l e the interstitial occurrence of a mineral against the crystal faces of another, does not mean that they are not involved, at least partially, in a cotectic relationship A remarkable, seldom described, and unexplained texture is the relatively common occurrence of ophitic quartz, where round or sub hexagonal domains of interstitial quartz incorporate laths of plagioclase (and other minerals) This feature occurs in situations where magmas of contrasting composition coexisted and most probably mixed, for instance in mafic microgranular enclaves incorporated in granitic rocks (Mont Blanc granite, Fully ana¬ tectic granodiorite, Western Alps, M Capanne granite, Italy) Mixing has, according to this view, led to intermedi ate compositions with the estabhshment of thermal equilibration between the still crystallizing mafic enclaves and acidic host The last stages of crystallization were completed at a low rate of nucleation, allowmg the development of large poikilitic quartz crystals enclosing laths of plagioclase, the marginal zones of which are coeval with the quartz b) Quartz also occurs as ocelh very commonly in tonahtic enclaves (within gramtoids) or in larger masses Although these have been considered to be early, high pressure precipitates, there is again very good evidence for magma mixing, and for the incorporation of quartz xenoerysts from a crystallizing acidic magma mto a more mafic liquid These quartz crystals are invariably rounded, even embayed, and are either rimmed with ferromagnesian minerals (biotite and/or hornblende), or not The nature of the ferromagnesian minerals, as well as the presence and the width of the nm depend, to a large extent, on the characteristics of the host material at time of incorpora¬ tion The nm may protect the quartz from further corrosion, but in any case some silicic liquid must be available and these mim magmas could also crystallize in the form of interstitial quartz The previous texture may be com bined with this one to form interstitial patches of quartz spreading out from the quartz ocelh In these occurrences, it can be clearly seen that the ocelh have an outer nm in optical continuity with the central part, but distinguish- able from the latter mainly through the presence of abundant long apatite needles Both ocelh and ophitic quartz exist in certain basalts, lamprophyres and appimtes as well c) Quartz, as well as feldspars, may also form interstitial patches directly projeeting from host granitoid into mafic dioritic enclave This suggests that gramtic liquid was capable of penetratmg the enclave to some degree, which may be partly enhanced by the development of a porous texture in the crystallizing enclave, due to some 10% reduetion in volume In conclusion, a number of features concerning the texture of quartz in diorites and tonalites are related to sit¬ uations in which the mixing of magmas very likely occurred The intermediate compositions may in fact be at least partly due to the mixing of two liquids of contrasting composition