Abstract

Microveins within granitic igneous bodies can form at all stages of crystallization from early magmatic to late post-magmatic and can be described as either inter- or intragranular. The indigenous character of such veins is evident when they begin and end within the scale of the thin section, but for larger dikes and veins an indigenous origin with respect to a parent pluton may or may not be apparent. The microveins result from mechanical activity, such as fracturing, affecting the system at the various stages of crystallization. The mineralogical character of the microveins is dependent on the timing of "mechanical release" with respect to the stage of consolidation of a specific portion of the water-bearing magmatic system. Overlapping relationships of dikes and veins characterizing the various stages are to be expected: in a specific portion of a pluton repeatedly affected by incomplete mechanical release of fluid as crystallization of that portion of the pluton proceeds or as the result of the simultaneous release of fluids from one portion of a pluton and the relocation in another in which release is occurring at a different stage of consolidation.The mineralogy of dikes and veins reflects the timing of separation of fluid from the crystallizing parent system, but it is somewhat variable depending on whether or not further release occurs from the crystallizing of newly separated fluid. Intergranular fluids of crystallizing H 2 O-bearing granitic magmas are classified as: main magmatic, late magmatic, early post-magmatic, and late post-magmatic. Main magmatic stage microveins contain minerals in equilibrium with fractionated intergranular melt existing at this stage of release from the crystal-melt parent system. The assemblage typically will be quartz, K-feldspar, plagioclase, and biotite, but it may include lesser amounts of late crystallizing K-feldspar, sodic plagioclase, quartz, and myrmekite if the system remains closed. If fluids of the main magmatic stage are accumulated and concentrated beyond the microscopic scale, for example, in dilational joints forming in an interlocking crystal framework, they form dikes comagmatic with the parent pluton.Late magmatic indigenous microveins typically contain quartz, K-feldspar, sodic plagioclase, and relatively large amounts of myrmekite, all of which crystallize from melt saturated in H 2 O. They originate from the parent pluton or by release from already separated, main magmatic-stage veins or dikes. Larger scale equivalents are leucogranites or alaskites; if internal segregation takes place, they become aplite-pegmatite complexes. Post-magmatic activity is dominated by the presence of H 2 O-rich fluids. Early post-magmatic indigenous microveins and their larger scale vein equivalents are characterized by secondary K-feldspar and quartz (+ or - ore minerals) and represent growth in an H 2 O-rich environment into which growth materials are entering from coexisting magma. In a closed aplite-pegmatite system the early post-magmatic stage is represented by the coarse-grained pegmatite fraction. Late post-magmatic indigenous microveins and their mesoscopic equivalents may contain ore minerals, quartz, and many other minerals, all of which crystallize from the very latest stage, H 2 O-rich fluids, either issuing directly from a specific portion of a pluton in which release takes place only very late, or from crystallizing systems already released from the parent pluton at an earlier stage.

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