09:00 〜 09:15
[SMP22-01] Trace element geochemistry of mineral phases within the Archean olivine dolerite dykes, Dharwar craton, India: insights into the chemical characteristics of parental magma.
キーワード：Olivine dolerite dykes, Neoarchean, Dharwar craton, Mineral chemistry
The Dharwar craton of India is one of the oldest cratons on Earth and is well known for its Archean geological record. The tectonic setting and evolution of Dharwar craton in Neoarchean (~2.8-2.5 Ga) is still a subject of debate since several models are proposed, which include intracontinental rifting, plume-related magmatism or subduction tectonics [1,2,3]. Significant magmatic activities such as granitic intrusions, greenstone volcanism and mafic-ultramafic intrusions occurred during Neoarchean, altering the lithospheric framework of the craton. Basement tonalite-trondhjemite-granodiorite (TTG) and the volcano-sedimentary greenstone belts are cut across by mafic dyke swarms of varying ages. Olivine dolerite intrusions of ~2.7 Ga are scattered in the Western Dharwar [4], and the presence of olivine as the primary mineral phase suggests that they might have formed from primitive magmas originated from the mantle. The trace element variations in the maor mineral phases like olivine, plagioclase, pyroxene and opaque minerals are considered in the present study to evaluate the magma generation and differentiation process and its implications on the tectonic evolution of Western Dharwar craton.
The olivine dolerite dykes show poikilitic textures and are composed primarily of olivine and plagioclase feldspar with lesser amounts of pyroxene and opaque minerals. Olivine grains are homogenous and devoid of compositional zoning suggesting the derivation from less fractionated melt. However, oxide minerals are sometimes in the core and along the rim of the resorbed olivine. The core-to-rim composition of olivine varies between Fo=83.3 and 69.4, the high Mg core preserving the parent melt composition. Orthopyroxene and clinopyroxene show a wide range of compositions. Diopside and augite are the major clinopyroxenes whereas hypersthene and a few ferrosilite compositions are obtained for orthopyroxenes. This is also reflected in the Mg# (mol), which ranges between 86 and 55. Plagioclase laths of size up to 2mm show typical polysynthetic twinning and a composition of An=77.1 to 62. The olivine-melt thermometry calculations [5] yield a temperature of 1391-1364oC for this suite of dykes. The whole rock geochemical characteristics show a depleted mantle source with an exceptionally low initial Sr isotope ratio (0.70088). This together with negative epsilon Nd values are interpreted as a mantle source with oceanic crustal components. The multiple sulfur isotope characteristics suggests a weak of mass independent fractionation (MIF)[6]. Negative 33S and 34S delta values are linked to an early formed mantle reservoir with a possible mixture of sulfur from subducting oceanic sediments. This also points towards the subduction-related crustal recycling and interaction of the in the Neoarchean.
References:
[1]Chadwick et al., 2000, Precambrian Res.[2] Jayananda et al., 2020, Gondwana Res.[3] Manikyamba and Kerrich, 2011, Precambrian Res.[4] Silpa et al., 2021, Lithos[5]Sisson and Grove, 1993, Contrib. to Mineral. Petrol[6]Silpa and Satish-Kumar, 2023, Lithosphere.
The olivine dolerite dykes show poikilitic textures and are composed primarily of olivine and plagioclase feldspar with lesser amounts of pyroxene and opaque minerals. Olivine grains are homogenous and devoid of compositional zoning suggesting the derivation from less fractionated melt. However, oxide minerals are sometimes in the core and along the rim of the resorbed olivine. The core-to-rim composition of olivine varies between Fo=83.3 and 69.4, the high Mg core preserving the parent melt composition. Orthopyroxene and clinopyroxene show a wide range of compositions. Diopside and augite are the major clinopyroxenes whereas hypersthene and a few ferrosilite compositions are obtained for orthopyroxenes. This is also reflected in the Mg# (mol), which ranges between 86 and 55. Plagioclase laths of size up to 2mm show typical polysynthetic twinning and a composition of An=77.1 to 62. The olivine-melt thermometry calculations [5] yield a temperature of 1391-1364oC for this suite of dykes. The whole rock geochemical characteristics show a depleted mantle source with an exceptionally low initial Sr isotope ratio (0.70088). This together with negative epsilon Nd values are interpreted as a mantle source with oceanic crustal components. The multiple sulfur isotope characteristics suggests a weak of mass independent fractionation (MIF)[6]. Negative 33S and 34S delta values are linked to an early formed mantle reservoir with a possible mixture of sulfur from subducting oceanic sediments. This also points towards the subduction-related crustal recycling and interaction of the in the Neoarchean.
References:
[1]Chadwick et al., 2000, Precambrian Res.[2] Jayananda et al., 2020, Gondwana Res.[3] Manikyamba and Kerrich, 2011, Precambrian Res.[4] Silpa et al., 2021, Lithos[5]Sisson and Grove, 1993, Contrib. to Mineral. Petrol[6]Silpa and Satish-Kumar, 2023, Lithosphere.