Synkinematic emplacement of granitoids in a Pan-African shear zone in Central Cameroon
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The Guandiping biotite diorite outcrops in the Mianlue suture zone,located in the western part of the Guangtoushan granite pluton.The biotite diorite is mainly consisted by plagioclase,bhrnblende and biotite,with slight alteration.LAICP-MS dating of zircon U-Pb yeilds a weighted mean age of 220.5Ma±3.1Ma(MSWD)=0.66,2σ),which is considered representing the crystallizing age of the Guandiping diorite pluton.This age is similar with the emplacement age of the biotite-plagioclase granite(216Ma±2Ma)from the Guangtoushan Pluton in the surroundings,it can be inferred that the Guandiping biotite diorite pluton was formed in post-collisional setting.
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Petrological, isotopic, and gravity data for a massive granodiorite with K-rich biotite-diorite and tonalite variants in the New England Batholith, Australia, indicate that the more mafic rock types form the lower parts of a pluton that has been tilted so as to expose the base along its southwestern margin. The mafic rocks are inferred to be cumulates of refractory and phenocrystal phases and not the products of independent K-rich basic magmas. This evidence of tilting in the unfoliated granodiorite Uralla pluton suggests that granite samples should be used cautiously in palaeomagnetic-palaeolatitude studies, for the absence of metamorphic foliation is not sufficient evidence that the granite has not been tilted.
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At Croft Quarry, exposures of a pluton belonging to the South Leicestershire Diorite suite have revealed a complex history of multiple intrusion. Soon after emplacement of the main-stage quartz-diorite, the partially crystallised pluton received an influx of magma which became dispersed and is now seen as partially assimilated dioritic xenoliths. A much later episode of intrusion occurred when the pluton had cooled sufficiently to be capable of fracturing. It resulted in a spectacular swarm of synplutonic quartz-diorite/tonalite sheets with contacts indicating that the host quartz-diorite was locally remobilized, disrupting and net-veining the later sheets. These features are typical of ‘magma mixing’ phenomena, and suggest an underlying process that may account for some of the geochemical and petrographical variations previously noted within the South Leicestershire diorites.
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The Cretaceous Lee Vining Diorite (eastern Sierra Nevada) preserves important field, petrographic and geochemical evidence for complex intrusive events in a mesozonal magma body. The pluton comprises primarily light grey unfoliated quartz diorite, with color index varying between 10--50. A 1,400 m transect across the pluton yielded samples of (1) mafic diorite from a 300 m thick sill, (2) disaggregated basaltic dikes, (3) rounded mafic inclusions and (4) rounded hornblende-cumulate inclusions in addition to (5) quartz diorite of variable texture and color index. Internal intrusive contacts are observed in several places; the upper contact of the sill is indistinct locally, suggesting a low thermal contrast with host quartz diorite at the time of intrusion. Textural analysis of plagioclase crystals form throughout the pluton indicates a history of pervasive magma mixing. In each thin section, 20--90% of the plagioclase crystals are pitted or corroded, and up to 50% of these crystals are also completely zoned. Disequilibrium plagioclase crystals are not preferentially associated with mafic inclusion, but are distributed randomly within the pluton. Major and trace element analyses of over 40 samples taken along the transect show variations that cannot be explained through simple magma mixing or through progressive crystallization of amore » single magma body. The pluton is not zoned geochemically, but rather comprises small regions (< 200 m across) of uniform composition that are juxtaposed randomly. Chemical variations across the pluton likely result from both missing of small magma batches and fractional crystallization of mixed magmas. This interpretation is consistent with field and textural requirements for multiple intrusive episodes in the formation of the diorite pluton.« less
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Abstract Petrology and P–T estimates indicate that a magmatic epidote‐bearing quartz diorite pluton from Mt. Gamsby, Coast Plutonic Complex, British Columbia, was sourced at pressures below ∼1.4 GPa and cooled nearly isobarically at ∼0.9 GPa. The P–T path indicates that the magma was within the stability field of magmatic epidote early and remained there upon final crystallization. The pluton formed and crystallized at depths greater than ∼30 km. REE data indicate that garnet was absent in the melting region and did not fractionate during crystallization. This suggests that the crust was less than or equal to ∼55 km thick at 188 Ma during the early phases of magmatism in the Coast Plutonic Complex. Late Cretaceous contractional deformation and early Tertiary extension exhumed the rocks to upper crustal levels. Textures of magmatic epidote and other magmatic phases, combined with REE data, can be important for constraining the P–T path followed by magmas.
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