Contact Metamorphic Assemblages Research Articles

Reworking of deep‐seated gabbros and associated contact metamorphosed paragneisses in the southeastern part of the Seiland Igneous Province, northern Norway

ABSTRACTThe Seiland Igneous Province of the North Norwegian Caledonides consists of a suite of deep‐seated rift‐related magmatic rocks emplaced into paragneisses during late Precambrian to Ordovician time. In the south‐eastern part of the province, contact metamorphism of the paragneisses and later reworking of intrusives and associated contact aureoles have resulted in the development of three successive metamorphic stages. The contact metamorphic assemblage (M1) Opx + Grt + Qtz + Pl + Kfs + Hc + Ilm ± Crd is preserved in xenolithic rafts of paragneiss within metagabbro. Geothermobarometric calculations yield 930‐960d̀ C and 5‐6.5 kbar for the contact metamorphism. M1 was followed by cooling, accompanied by strong shearing, formation of the gneiss foliation and recrystallization at intermediate‐P granulite facies conditions (M2). Stable M2 phases are Cpx + Opx + Pl +Ilm ± Hbl in metagabbro and Grt ± Sil ± Opx + Kfs + Qtz + Pl ± Bt + Ilm in host paragneiss. The M2 conditions are estimated to 700‐750d̀ C and 5‐7 kbar. A subsequent pressure increase is recorded in the M3 episode, which is associated with recrystallization in narrow ductile shear zones and secondary growth on M2 minerals. M3 is defined by the assemblages Grt + Cpx ± Opx + Pl + Ru + Qtz in metagabbro, and Grt ± Ky + Qtz + Pl ± Kfs + Bt + Ru in host paragneiss. M3 conditions are estimated to 650‐700d̀ C and 8‐10 kbar. The substantial pressure increase related to the M2 → M3 transition is interpreted to be a result of (early?) Caledonian overthrusting. Chemical zoning in cordierite and biotite suggest rapid cooling following the M3 event. The proposed P‐T‐t evolution implies that the tectonic evolution of the Seiland Igneous Province was long (at least 330 Ma) and complex and involved initial rifting and extension followed by crustal thickening and compression.

Structural controls on the emplacement of the vein-type tungsten-tin ore at Mount Carbine, Queensland, Australia

Formation of the vein-type tungsten-tin deposits at Mount Carbine, Queensland, is explained in terms of mineralization in favorable structural sites, created by three major deformations in the host succession of Devonian clastics. Upright folds with a vertical axial plane cleavage (S 1 ) formed during the first deformation (D 1 ), which was followed by conjugate kinking (D 2 ) of S 1 , where this cleavage is strongly developed, and by granitic intrusion. Contact metamorphic assemblages in the granite aureole were established before local recumbent folding (D 3 ) of S 1 . At late stages of this latter event, vein mineralization developed along D 2 kink planes in sites of high D 1 strain, where capped by domains of D 3 folding.Evolution of this mineralization can be recognized at the Mount Carbine mine. Early stages of fracture dilation are marked by tourmaline infill and associated wall-rock alteration, with fibrous crack-seal microstructures indicating repetitive tensile failure and tourmaline deposition. Subsequent major dilations are marked by veins of nonfibrous quartz and feldspar, and ore-grade wolframite. In similar mineralizations in nearby prospects, these veins merge with greisen veins and granite dikelets, supporting a granite hydrothermal model for the origin of the tin and tungsten in the area.The late D 3 vein mineralization was, however, preceded by the local formation of a differentiated layering (S 3 ), and of sets of barren veins along S 3 and the D 2 kink planes. Both the layering and barren veins postdate the growth of the contact metamorphic assemblage around the undeformed granite.

Tilted plutons in the Klamath Mountains of California and Oregon

Geologic and petrologic evidence indicates that the Ashland pluton and the Wooley Creek batholith, two major plutons of the Klamath Mountain province, have been significantly tilted subsequent to emplacement. Late Cretaceous sandstone depositionally overlies the Ashland pluton and dips approximately 25° toward the ENE. Contact metamorphic assemblages in diverse wall rocks surrounding the Wooley Creek batholith suggest at least 30° of tilt toward the WSW. Subsequent erosion has exposed at least 12 km of structural relief in the Wooley Creek batholith. Tilting was accomplished, at least in part, by thrusting of the plutons and their host rocks over low‐density rocks of the western Jurassic belt, followed by regional doming of deeply buried rocks of the underthrust terrane (Condrey Mountain Schist). Diagrammatic reconstruction of the plutons to pretilt positions suggests inward chemical fractionation of the Ashland pluton but upward fractionation of the Wooley Creek batholith and comagmatic Slinkard pluton. The results of this study indicate that rootless plutons in accretionary margins can undergo significant tilting, facilitating petrologic interpretations. In addition, the study suggests that for Klamath plutons, the effects of tilting must be taken into account before meaningful paleomagnetic interpretations can be made.

Phase Relationships in the System Fe3Al2Si3O12-Mg3Al2Si3O12-H2O at 2.0 Kilobars

The phase relationships in the system Fe 3 Al 2 Si 3 O 12 -Mg 3 Al 2 Si 3 O 12 -H 2 O (almandine-pyrope-H 2 O) were investigated at 2.0 kb fluid pressure, under controlled oxygen fugacities defined by the fayalite + magnetite + quartz buffer. The stable garnet in the range of temperatures investigated is essentially almandine; the maximum pyrope content found at 648° C was 22 mole percent. The phase relationships are strongly influenced by bulk composition, especially near that of almandine. At high temperatures, garnet coexists in equilibrium with quartz + 2 spinels, cordierite + quartz + spinel, and cordierite + olivine + spinel; at lower temperatures, it is in equilibrium with quartz + chlorite + spinel, cordierite + chlorite + quartz, cordierite + orthoamphibole + chlorite, and cordierite + orthoamphibole + spinel, both in the order of increasing pyrope component in the bulk composition. In the compositional range where garnet is not stable, the anhydrous assemblage, cordierite + olivine + spinel, is in equilibrium with the hydrous equivalent, cordierite + chlorite + olivine which, in turn, coexists with the more hydrous assemblage, cordierite + chlorite + talc. At temperatures above 800° C, olivine from bulk compositions richer in pyrope component than Am 65 Pr 35 gives way to orthopyroxene which has a limited compositional range. In all cases, garnet is more ferruginous than the coexisting ferromagnesian silicates. The results indicate that only an iron-rich chlorite is stable in rocks that contain an almandine-rich garnet and quartz. They also indicate that the contact metamorphic assemblage, cordierite + anthophyllite + almandine-rich garnet, has a large field of stability at 2 kb fluid pressure.