LAJINHA, UMA INTRUSÃO TARDI-OROGÊNICA E PÓSCOLISIONAL NO EXTREMO OESTE DO COMPLEXO PARAÍBA DO SUL, MINAS GERAIS, ESPÍRITO SANTO, BRASIL

Abstract

This is an interpretation of geological, petrological and geochemical data of the Lajinha intrusion,which belongs to the Alto Capim Subsuite of the Espírito Santo Magmatic Suite. The Lajinhaintrusion is a plutonic body of medium extensions (48km2) with diapiric structure and magmaticzoning. Two magmatic groups build up this body, one of granitic affinity and the other group ofdioritic affinity. Between this two groups do not exist “commingling” but a chemical interaction (e.g.: “mixing” in the contact region). The acid rocks, that crop out in a ringlike form, can be observedin two distinct litological groups. The main group is coarse-grained, has a great number of big potassicfeldspar-phenocrysts (up to 15cm), small part of matrix and it do not have a preferencial orientation.In some parts exist a flux orientation of the phenocrystals and at the contact with the host rockapears weak intrusive foliation. The other subordinate group is formed by a middle to coarse grainedmatrix, with isotropic apearence and with a very low content of K-feldspar phenocrystals. Theseparation of the two groups may be a result of “filter-pressing” and/or “gravity separation”. Thegranites are constituted principally of quartz, K-feldspar, plagioclase, biotite, accessory and opaqueminerals. Their chemical composition ranges from alkaligranite to granodiorite. The basic rockgroup,which crops out in an oval formed elonged central nuclei, has a middle to coarse granulometry,homogeneous and isotropic apearence over all of the visited outcrops. This rock is built up byamphibole, biotite, plagioclase, small percentage of pyroxene, accessory and opaque minerals. Thechemical composition vary between syeno-diorite, diorite and gabbrodiorite. The contact betweenthe granitc rockgroup and the basic rockgroup is very well defined and shows only metasomatismeffects and not “commingling”. The host of the intrusion are para-rocks like migmatites, bandedgneisses, quartzites, and ortho-rocks like an homogeneous gneisse (Galileia Intrusive Suite; PocraneComplex) and amphibolitic. They are very strong foliated. The contact between the host and theintrusive rocks is normally formed by gradational evolution (transformation) and in some parts ofthe north western contact by faults. From the host towards the intrusion growth up the number ofblasts and the distribuion of fusion effects. Pegmatitic veins cut the mainbody in the same directionas distension fractures (NE-SW). In the northeastern part this fractures are invaded by a fine grainedalkaline basalt of probably cretaceous age. This veins cut the host and the granites of the intrusion,too. The observations make possible the idea of an intrusive process under late-orogeneous to postcollisionalregime. The differentiation trends of the two rock groups are calco-alkaline (trondhjemiticto monzonitic), with peraluminous to metaluminous and alkaline to subalkaline tendency. Thisintrusion make part of the symetric body distribution within the Espírito Santo Magmatic Arc,beginning with late to post-collisional intrusion in the west and east and early to syn-collisionalintrusions in the central region, the later influenced by compressional and transcorrent movements.