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Petrological Characteristics and Genesis of the Central Indian Ocean Basin Basalts

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Petrological Characteristics and Genesis of the Central Indian Ocean Basin Basalts Page | 1 Author version: Acta Geol. Sin., vol.86(5); 2012; 1154-1170 Petrological characteristics and genesis of the Central Indian Ocean Basin basalts PRANAB DAS1, SRIDHAR D. IYER1 AND SUGATA HAZRA2 1NATIONAL INSTITUTE OF OCEANOGRAPHY (COUNCIL OF SCIENTIFIC AND INDUSTRIAL RESEARCH) DONA PAULA, GOA – 403004 INDIA 2SCHOOL OF OCEANOGRAPHIC STUDIES, JADAVPUR UNIVERSITY KOLKATA – 700 032 INDIA ABSTRACT The Central Indian Ocean Basin (CIOB) basalts are plagioclase rich while olivine and pyroxene T are very few. The analyses of forty five samples reveal high FeO (~10-18 wt%) and TiO2 (~1.4-2.7 wt%) indicating these a ferrobasaltic affinity. The basalts have typically high incompatible elements (Zr 63-228 ppm; Nb ~1-5 ppm; Ba ~15-78 ppm; La ~3-16 ppm), a similar U/Pb (0.02-0.4) ratio as the N- MORB (0.16±0.07) but the Ba/Nb (12.5-53) ratio is much higher than that of the normal mid-oceanic ridge basalt (N-MORB) (~5.7) and Primitive Mantle (9.56). Interestingly almost all of the basalts have negative Eu anomaly (Eu/Eu* = 0.78-1.00) that may have resulted by the removal of feldspar and pyroxene during crystal fractionation. These compositional variations suggest that the basalts were derived through fractional crystallisation together with low partial melting of a shallow seated magma. Key words: Basalts; CIOB; genesis Page | 2 1. INTRODUCTION The Indian Ocean is exemplified by the Central Indian Ridge (CIR), Southwest Indian Ridge (SWIR) and Southeast Indian Ridge (SEIR). The Central Indian Ocean Basin (CIOB) extends from the Ninetyeast Ridge in the east to the south of the Chagos–Lacaddive Ridge and the CIR in the west and is bounded in the south by the Rodriguez Triple Junction and the northern part of the SEIR and in the north by India and Sri Lanka. The half spreading rate of the CIOB crust has been recorded to be 80 mm/yr (McKenzie and Sclater, 1971) and 80 to 20 mm/yr (Patriat and Segoufin, 1988). Kamesh Raju and Ramprasad (1989) documented that during A25 to A23 the average rate was 80 mm/yr but decreased to an average of 36 mm/yr during A23 to A21. In contrast, Mukhopadhyay et al. (1997) reported a rate between 80 and 36 mm/yr while Rajendran and Rao (2000) suggested an average rate of 78 mm/yr. Dyment (1993) more precisely calculated the spreading rate as 68 mm/yr to 92 mm/yr till A24 that decreased to 45 mm/yr after A24. Hence, it is evident that the CIOB has witnessed several episodes of spreading at variable rates. The CIOB (avg. water depth 5100 m, 4500-5600 m) hosts several morpho-tectonic features like the trace of triple junction on the Indian Plate (TJT-In, Dyment, 1993), fracture zones (FZ), seamounts and lineations (Fig. 1) that have notably affected the stability and volcanic activities of the basin. Dyment (1993) suggested that propagating rifts may have influenced the evolution of the TJT-In during chrons A28 to A21 (~68 to 50 Ma), the approximate time when the CIOB was formed. Bathymetry reveals abundant isolated seamounts and seamount chains sub-parallel to each other and to the major FZ (73° E, 79° E and 75°45′ E). A 200 seamounts occur either as isolated edifies or along eight sub-parallel chains, that trend almost N-S and probably formed from the ancient propagative fractures. A majority of these near-axis seamounts may be the products of the temporally widespread (Cretaceous ~65 Ma to late Eocene <49 Ma) collision between India and Eurasia. The mutual effect of the regional stress patterns retained the orientation of the chains while the local stress regime aided upwelling of magma and construction of the seamounts. Evidences indicate that the morphotectonic structures developed concurrently with the formation of the oceanic crust (Das et al., 2007). Iyer and Karisiddaiah (1990) reported the characteristics of the basalts and later Iyer (1995), Mukhopadhyay et al. (1995) and Das and Iyer (2007) identified these as normal mid-oceanic basalt (N- MORB) while spilites occur sporadically (Karisiddaiah and Iyer, 1992). In the Indian Ocean, Page | 3 ferrobasalts have been recovered from Deep-Sea Drilling Project (DSDP) Sites 214, 216 and 254 on the Ninetyeast Ridge, Site 256 from the Wharton Basin (Thompson et al., 1978), Southeast Indian Ridge (SEIR) (Anderson et al., 1980), the Southwest Indian Ridge (SWIR) (le Roex et al., 1982) and the Australian–Antarctic Discordance (Klein et al., 1991). Iyer et al. (1999) reported ferrobasalts to occur in areas of morphological highs and enhanced magnetic amplitude in the CIOB. Here we present the geochemical characteristics of the CIOB basalts and evaluate and interpret their distinctive features that may shed light on the mantle source and the conditions responsible for melt generation. We draw attention to the fact that this study pertains only to the basalts collected from the seafloor while those from the seamounts would be dealt later. 2. MATERIALS AND METHODS A majority of the recovered samples are fragments and pillow basalts with slightly to highly altered glassy layer. For this study samples with no glass or with a very thin veneer were selected. To avoid the alteration effects we removed the glass and in the latter case selected the interior part of the samples. Forty-five samples were sliced using a diamond embedded saw and then abraded with sandpaper to remove the saw trace and remaining visible alteration. The fresh samples were cleaned using acetone and distill water and were coarsely crushed in a hydraulic piston crusher before powdering in a tungsten-carbide ring mill. To minimise contamination effects all the samples were powdered for 2 minutes. Major element oxides concentrations were determined by using a X-ray fluorescence (XRF) following the analytical procedure described by Rhodes (1996). The minor, trace and rare earth elements (REE), were determined through an inductively coupled plasma-mass spectrometry (ICP-MS, Perkin Elmer’s Elan DRCe and Perkin Elmer’s Elan DRC-II). The sample powder (0.05 gm) was digested with a mixture of HF, HNO3 and HClO4 to remove silica. Diluted HNO3 was added to dissolve the digested material and later distill water was added to make a 100 ml aliquot for use with the ICP-MS. A blank solution was run for each set of five samples and the reading was used to correct the results for contamination (if any) during solution preparation. Repeated measurements of BHVO-1, BRC-1 and JB- 2 were made for calibration and calculation of the accuracy of the analysis. The relative standard deviations for the major oxides was < ± 1% while for the trace elements and REE analysis it was ±0.6 to 3%. The mineral analyses were performed with Cameca SX100 microprobe. For the analysis, the operating voltage was 15KeV a probe current of 20 nA was used, and the beam diameter was 5 μm. Page | 4 Calibration was carried out using natural mineral standards (BRGM, Orleans Cedex, France). Precision was better than 5% for each element and counting time was 10s for each element. The obtained data were ZAF corrected internally to eliminate all possible element interferences (after Philibert, 1963). Semiquantative energy-dispersive spectra analysis (EDS) of five plagioclasen grains were carried out with a JEOL SEM-EDS link system (JEOL, Tokyo Japan) at the National Institute of Oceanography, Goa, India and rock magnetic studies were conducted by using a Mole spin instruments at Indian Institute of Geomagnetism, Alibagh, India. 3. RESULT AND INTERPRETATION 3.1. Hand specimens Several dredging operations have been performed in the CIOB and a variety of rocks have been recovered (Fig. 1; Table 1). A total of forty five samples were examined during the course of this work. The rocks are pillow lavas, with the outer rind made of glass (Fig. 2). Altered glass is seen in a few basaltic fragments. The small fragments of glass/ basalt chips were probably dislodged from larger outcrops. Most of the rocks are either sparsely phyric or aphyric basalts. In some samples plenty of vesicles of irregular to rounded shapes are noticeable. Not much variation is apparent in hand specimens among the samples. 3.2. Petrography Mineralogically CIOB basalts are plagioclase phyric basalts. A majority of the basalts have plagioclase as a dominant mineral phase while olivine and pyroxene occur as subordinate minerals. Plagioclase occurs as phenocryst and acicular grains as a constituent of ground mass and some of the grains have jagged ends and corroded margins. Plagioclase phenocryst exhibits prominent lamellar and cross-hatched twinning (Fig. 3a) while sector zoned plagioclase is rare (Fig. 3b). A few vesicles occur and these are mainly rounded in shape sometimes have secondary minerals. Plagioclases occurring as phenocrysts or microlites are quite fresh. Plagioclase phenocrysts showing fracturing and wavy extinction and rarely, fine tiny and spherical inclusions are observed within the plagioclase phenocrysts. At places two sets of lamellar twinning are present in a single crystal (Fig. 3c). In places, plagioclase phenocrysts with prominent crystal outline on three sides and corroded on one side are observed. Plagioclase phenocrysts with basaltic groundmass extending and into the twin lamellae are noticeable. Plagioclase phenocrysts showing reaction margin with the glassy groundmass Page | 5 are common. A few phenocrysts of plagioclase show both twinning and zoning (Fig. 3d) while, grains showing reaction relations with the glassy groundmass are also present (Fig. 3e). Generally, olivine grains are altered to iddingsite (brown colour mineraloid) and fractured. In one sample unaltered euhedral olivine occurs in a glassy matrix (Fig. 3 f). Olivine grains exhibit inclusion of the plagioclase laths into them. The groundmass consists of acicular plagioclase (may be due to the effect of quenching) exhibiting radiating fibrous structure.
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