SUCCESSIVE MIXING AND MINGLING OF MAGMAS IN A PLUTONIC COMPLEX OF NORTHEAST BRAZIL

Field and petrographic evidence together with major element geochemist ry suggest that mixing and mingling of magmas of contrasting compositi ons were important petrogenetic processes in the Fazenda Nova/Serra da Japeganga plutonic complex of Northeast Brazil. The complex was empla ced at pressures of 300-500 MPa in amphibolite facies metamorphic rock s of Neoproterozoic age and consists of three main rock types: (1) coa rse-grained granite; (2) porphyritic granite and (3) diorite to quartz -monzodiorite. The latter two make up the Fazenda Nova batholith which is located on the northwestern side of the sinistral, NE-trending, Fa zenda Nova strike-dip shear zone. NE-plunging stretching lineations in the shear zone suggest that this batholith represents an uplifted, an d therefore deeper, portion of the complex. The structure of the compl ex reflects the stratigraphy in a magma chamber, with the porphyritic granite above the diorite and below the coarse-grained granite. The po rphyritic granite has a uniform composition, intermediate in mafic min eral content, quartz, and major-elements between the coarse-grained gr anite and the diorite. It is free of disequilibrium mineral assemblage s, and locally displays gradational contacts with the overlain coarse- grained granite. Most elements display linear correlation with SiO2 in Harker diagrams. These features are interpreted as resulting from mix ing of almost crystal-free felsic and intermediate magmas. Fluid dynam ic calculations using the coarse-grained granite and the silica-poores t diorite as end-members in the mixing process show that mechanical mi xing was possible, and thermal modelling suggests that the formation o f an homogeneous hybrid may have been achieved in less than 50,000 yr. The diorites contain corroded K-feldspar megacrysts, and range in com position from low to relatively high silica contents, partly overlappi ng with the porphyritic granite. This suggests that a new mixing event occurred during the crystallisation of the porphyritic granite, this time producing a heterogeneous, xenocryst-bearing, dioritic hybrid. Ab undant enclaves of diorite in the porphyritic granite, despite their t extural diversity, are typically devoid of chilled margins, and were t herefore formed relatively early in the crystallisation history of the granite. They are interpreted as liquid droplets separated from the h eterogeneous hybrid magma through convection currents and incorporated in the crystallising granitic magma. Subsequently, during the crystal lisation of the porphyritic granite, mafic magma supply to the batholi th continued at a declining rate, probably assisted by the development of the Fazenda Nova shear zone. This leads to the production of strom atitic-like structures, with alternating bands of mutually contaminate d granite and diorite, then to the intrusion of contorted synplutonic dykes, and, finally, of late-stage dykes, some of which with chilled f iner-grained margins.