Garnet composition in the Indus Fan core sediments from IODP Expedition 355 (Arabian Sea)

The thesis is moving now to the study of garnets in the Indus Fan as case history.
The western Himalaya provides an opportunity to study what processes control continental erosion (climate, tectonic and topography). Because of rapid transport, detrital signatures of modern sand faithfully reflect the lithology of source terranes (Garzanti et al., 2005). The sedimentary record is a detailed archive of past erosion that may be used to test which processes dominate over different time scales.
In total, 205 grains of garnets, spread between the five samples, had been deeply analysed, using Raman spectroscopy, in order to reconstruct the source rocks based on the ternary plot of Mange e Morton (2007).

Geographical framework
The geographical position (figure 10) of the core drilled during the IODP Expedition 355 is the Laxmi Basin, located in the eastern Arabian Sea. More specifically, it is located between a continental block, called Laxmi Bridge (on the left side) and the west coast of India on the right side. The ocean here is around 3.5 km depth, and this is the reason to choose the Laxmi Basin: due to it is located next to the border of the Indus Fan, it is thinner than the core. The probability of drilling older information is higher here than in the centre of the Indus Fan. However, in the cored Laxmi Basin succession, heavy minerals are far less affected by selective diagenetic dissolution.
The core U1456 is situated here, in the second largest sediment body on Earth. The sediments eroded from Western Himalaya have been here deposited but before being accumulated, they have been transported for thousands of kilometres by the Indus River and its tributaries; they represent the primary source of clastic sediments into the Arabian Sea.
The Indus River, sourced from the central southern Tibetan Plateau, is around 3000 km long and has a basin area of around 106 km2. It flows from Tibet to the Arabian Sea mostly through arid land.

Geological setting
The geological area (figure 11) object of the thesis is divided into different geological units. The various mountain ranges are the source rocks of the sediments in the Indus Fan.
From north to south there is the Karakorum belt which includes the Northern Karakorum sedimentary domain, the Central Karakorum batholith, and the Southern Karakorum metamorphic belt (Searle et al., 1999). The Ladakh and Kohistan Arcs: they are batholiths representing the dissected remnants of magmatic arcs fed by northern subduction of Neotethyan lithosphere during the Cretaceous to earliest Paleogene. The Kohistan arc is composed of six main units; the Ladakh batholith consists of a suite of mafic to felsic rocks (Weinberg et al., 2000). This is separated by the Indus Suture Zone from the Himalaya belt. The Tethys Himalaya, the Greater Himalaya, the Lesser Himalaya and the Sub-Himalaya are included in the Himalayan range. Himalaya is the largest mountain belt on Earth. It is the result of the collision between the Indian passive margin and the Asian active margin that started about 50-60 Ma ago (Hu et al., 2016). Tethys Himalayan succession consists of siliciclastic and carbonate rocks (Sciunnach et al., 2012); Greater Himalaya includes slate intruded by granitoids and sillimanite-bearing metasedimentary rocks at the top (Di Pietro & Pogue, 2004); Lesser Himalayan includes the basement; Sub-Himalayan rocks include cover strata displaying southward decreasing metamorphic grade (Greco & Spencer, 1993). The Indus River drains all these domains until it reaches the Arabian Sea.

The IODP Expedition 355
The International Ocean Discovery Program (IODP) is one of the biggest international programs about Earth sciences. Its focus is related to marine research and it has the aim to deeply study data recorded during the expeditions. The samples that could be collected are made of sediments, rocks, fluids, and living organisms from below the seafloor. It monitors climate and ocean changes reading the past, the Earth in motion and the different samples that can be used for studying different scientific topics, such as provenance studies as in this elaborate.
The IODP Expedition 355 took place from the 31st of March 2015 to the 31st of May 2015 in the Laxmi Basin located in the Arabian Sea. Only one of the two sites drilled is the object of this study, the site U1456 (figure 13). The supervisor of this thesis, the professor Sergio Andò, had the pleasure to be part of the expedition team as sedimentologist and expert in heavy-mineral analyses.

The used methodologies
One of the aims of the thesis is to learn and improve the use of some methodologies. The chapter has the role to introduce and show the tools which had been used giving a quick brief about their operation.

Optical microscope
The optical microscope (figure 14) has been the first tool used to recognise the garnets in the five thin sections (figure 15). The maps (figures 17, 18, 19, 20, 21) of the five samples have red and pink circles around the garnet grains. Those are the grains later analysed by the Raman spectrometer. Those grains give us information about source rocks recording climate and tectonic events.
The information about all the samples can be seen in table 3, but it is important to give a better explanation of the sample 56E15. This sample belongs to the sediments deposited in the Indian passive margin and it represents the event of the big slumping described in the publication after the Expedition 355 and in Garzanti et al., 2020. The very little amount of garnets (only 12) provides a semiquantitative information and all these garnets are derived from the metamorphic basement of the Indian craton with high content of Mg (high grade of metamorphic rocks) compared with other garnets encountered in this study.