Petrogenesis and Geochronology of the late-Archean Na-rich A Type granite from the Bundelkhand Craton, India: Implication for tectonic and crustal evolution

<p>Tonalite-trondhjemite-granodiorite gneisses (TTG) are the oldest litho-units of the Bundelkhand craton. The supracrustal rocks include variable deformed mafic volcanics and Banded Iron Formation. Magmatic zircons from the TTG’s yield an upper intercept of ~ 3590 Ma. The TTG’s gradually grades to a Na-feldspar rich A type porphyric granite towards the south. In this abstract, we report mineralogical, geochemical, and geochronological information of high silica- low Ca - high Na A-type granite from Bundelkhand craton.</p><p>In the TAS diagram, the studied samples plot in the field of granite and have a metaluminous affinity with high Ga/Al and Ce + Y + Nb + Zr values typical of A-type granites. In a primitive normalized multi-element spider diagram, the studied samples exhibit negative Nb, Ti, and P anomalies characteristics of a subduction zone setting. The chondrite normalized REE’s exhibit a strong fractionated pattern with negative Eu anomaly; the LREE are enriched and the HREE depleted with moderate to high (La/Yb)<sub>CN </sub>ratios ranging from 11.12 to 26.24 ppm. The studied samples have plagioclase compositions that vary from X<sub>Ab </sub>= 0.980-0.997 and chlorite compositions varying from X<sub>Mg </sub>= 0.309-0.469.</p><p>Phase equilibria modeling yield an emplacement temperature of 700-750<sup>O</sup>C, at 1.0 GPa. Most of the zircon grains are prismatic with visible cores and rims in optical examinations. In a U-Pb concordia diagram, the grains yield an upper intercept of 2536.6 ± 1.8 Ma. The geochemical and geochronological data taken together, indicate the Na-rich A-type granite generated by the high temperature and high-pressure partial melting of Archaean supracrustal rocks.</p>

Emplacement Temperature of the Overbank and Dilute-Detached Pyroclastic Density Currents of Merapi 5 November 2010 Events using Reflectance Analysis of Associated Charcoal

Merapi eruption in 2010 produced 17 km high column of ash and southward pyroclastic density current (PDC). Based on the deposits characteristics and distributions, the PDC is divided into channel and overbank facies (pyroclastic flow), and associated diluted PDC (pyroclastic surge). The hot overbank PDCs and the associated dilute-detached PDCs are the main cause of high casualty (367 fatalities) in medial-distal area (5–16 km), especially near main valley of Kali Gendol. We reported the emplacement temperature of these two deposits using reflectance analysis of charcoal. We used both entombed charcoals in the overbank PDC and charcoals in singed house nearby. Samples were collected on 6–13 km distance southward from summit. Charcoalification temperatures of the entombed charcoals represent deposition temperature of the overbank PDCs, whereas those of charcoals in the singed house resembles temperature of the associated dilute-detached PDCs. Results show mean random reflectance (Ro%) values of entombed charcoal mainly range 1.1–1.9 correspond to temperature range 328–444 °C, whereas charcoal in singed house range 0.61–1.12 with estimated temperature range 304–358 °C. The new temperature data of the dilute-detached PDCs in the medial-distal area is crucial for assessing impact scenarios for exposed populations as it affects them lethally and destructively