Felsic rocks are minor in abundance but occur ubiquitously in International Ocean Discovery Program Hole U1473A, Southwest Indian Ridge. The trace element abundances of high-Ti brown amphibole, plagioclase, and zircon in veins, as well as the presence of myrmekitic texture in the studied felsic rocks support crystallization origin from highly-evolved melts, probably controlled by fractional crystallization. Based on geochemical criteria and texture of the mineral assemblage in felsic rocks and their relationship with host gabbros, they can be divided into three types: (1) Felsic rock with sharp boundaries is formed when felsic melt intrudes into fractures of host gabbros, resulting in minimal interaction between the melt and the wall minerals. (2) Replacive felsic rock, which is characterized by a pseudomorphic replacement of minerals in the host gabbro. This vein type is caused by the replacement of the host mineralogy by minerals in equilibrium with the felsic melts. (3) Felsic rock with diffused boundaries is formed either by infiltration of felsic melt into the solidifying gabbro body or crystallization of interstitial melts. Infiltration modes of felsic melts are likely controlled by the temperature condition of the cooling host gabbros.
Orogeneza indochińska, uznawana za zdarzenie kolizyjne pomiędzy płytami południowych Chin i Indochin, które miało miejsce w rejonie Azja Południowo-Wschodnia, w tym w Wietnamie, doprowadziła do powstania indochińskiego pasma górskiego. Jednak pytanie ilu ile razy w przeszłości doszło do zderzenia tych płyt i w którym okresie pozostaje bez odpowiedzi. Co więcej, zamknięcie starożytnego oceanu i ślady jego pozostałości pomiędzy płytami południowych Chin i Indochin są nadal przedmiotem poważnej debaty.
Pochodzenie indochińskiego termotektonizmu jest niepewne. Cały region został dotknięty procesami termotektonicznymi na zachodzie, powstały w wyniku zderzenia płyty Sibumasu z blokiem Indochin. Badanie orogenzezy przeprowadzono z wykorzystaniem paleogeografii, a także paleontologii oraz danych termochronicznych. Nie ma jednoznacznych dowodów sugerujących, że bloki Indochin i Chin Południowych zderzyły się w okresie triasu. Preferowany jest scenariusz płyt tektonicznych, który opisuje orogenezę indochińską jako zdarzenie reaktywacyjne wywołane akrecją bloku Sibumasu do Indochin.
Abstract Igneous microtextures are significant tracers of magmatic processes as these provide important information about magma evolution. Oxy-symplectite intergrowth of orthopyroxene (host) with Fe-Ti oxide (lamellae) is described from the Atlantis Bank, an Oceanic Core Complex (OCC) along the ultraslow spreading Southwest Indian Ridge (SWIR). This texture is widespread in the oxide-rich lithologies recovered from the lower crustal section of the Atlantis Bank. There exists a long-standing debate on the magmatic vs. metamorphic origin of the oxy-symplectites. The symplectitic intergrowth is usually noted wherever the olivine grain is in intimate proximity to the magmatic magnetite and is developed at the expense of the olivine. Comparative oxybarometric results between the ilmenite-magnetite pairs close to the symplectites and those away from the symplectites reveal relatively higher fO2 range for the former group. These observations along with phase equilibria modelling suggest that oxidation of olivine primocrysts may lead to the development of oxy-symplectite in the studied gabbros (sensu lato). Furthermore, the compositional variabilities between the symplectitic phases and the discrete magmatic phases away from the symplectite indicate that the origin of the oxy-symplectite took place under subsolidus condition. Additionally, phase equilibria modelling reveals that for the observed range of olivine compositions, higher oxidizing conditions are required for symplectite formation at higher temperatures. Synthesizing all the results, we suggest that the formation of the oxy-symplectite in this lower crustal section is a subsolidus process, where the oxide phases (especially magnetite) aided the oxidation of olivine.
The hornblende and biotite - bearing metagabbro of the Nui Ngoc complex were exposed into a few small blocks in the southwest Tam Ky city and were viewed as parts of the Tam Ky - Phuoc Son ophiolitic complex (TPO). These rocks were undergone mylonitic deformation and metamorphism. The rocks consist mainly of orthopyroxene, clinopyroxene, plagioclase, olivine, hornblende, biotite and very few microscopic Cr - spinel. The clinopyroxene and Cr - spinel minerals were analyzed for their composition by EPMA, the results showed that: (1) The clinopyroxenes have low Al2O3 (3,2÷3,5 wt %), TiO2 (0,70÷0,82 wt %) contents and (2) the Cr - spinels have low TiO2 (0,23÷0,58 wt %) content and Mg#100* (Mg/ (Mg+Fe2+)) (32÷42) but has medium Cr# (Cr/ (Cr+Al)) (45÷52).These features are similar to those of rocks formed in anoceanic - oceanicsubduction zone that was reported for the plagiogranite of Dieng Bong complex nearby. This study results combined with previous research results in the Tam Ky - Phuoc Son suture zone show the existence of island arc, continental arc, and MOR - magmatic types.
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