Dynamics of the Earth System: Evolution, Processes and Interactions by Unknown

Author:Unknown
Language: eng
Format: epub
ISBN: 9783030406592
Publisher: Springer International Publishing

5.13 Tectonic Setting

Carbonaceous sediments may be accumulated at various sites in passive and active margin setting (Dorobek 2008). The Lesser Antilles volcanic arc is an oceanic island arc with active margin conditions wherein American plate is being subducted below the Caribbean plate given rise to a chain of volcanic islands in an arcuate shape (Le Friant et al. 2013; Macdonald et al. 2000). Based on the geochemical characteristics of the studied sediments, the tectonic setting in which their deposition took place is although known to be an active continental margin setting, a verification has been made by using major oxide ratios K2O/Na2O versus SiO2/Al2O3 proposed by Roser and Korsch (1986; Fig. 10b). These discrimination diagrams have been used by many workers for texturally dissimilar rocks (coarse grained sandstones to med grained greywackes to fine grained shales/mudstones) which are derived from complex active margins including material derived from continental margin magmatic arcs (and deposited in a variety of basin settings including trench, fore-arc, intra-arc, and back-arc). On the other side when these discrimination diagrams were used for modern sediments which are chemically distinctive such as trailing-edge and forearc muds, but compositions of continental margin arc, strike-slip, and back-arc muds overlapped considerably. The studied carbonaceous samples when plotted on the K2O/Na2O versus SiO2/Al2O3 diagram occupy the field of A1–A2 (A1 = arc setting, basaltic and andesitic detritus; A2 = evolved arc setting, felsitic-plutonic detritus) very similar to the results obtained by various other geochemical ratios of the present study (Fig. 10b). Further, an online computer programme known as APM Disc (Active and Passive Margin Discrimination diagrams) for the discrimination of active and passive margins was written in Java using the framework ZK developed and published by Verma and Altrin (2016) is used for the studied samples. The compositional data of major oxides and trace elements (DF A-P MT) for the studied samples have been processed using the excel sheets and discrimination diagrams are obtained. Figure 9c shows that all the studied samples effectively discriminated with active margin affinity. Since the drilled core samples are collected from Lesser Antilles arc (a known active margin tectonic setting), the samples with <30 wt% SiO2 are also included during data processing (out of 20 samples 14 have SiO2 contents less than <30%) for APM Disc programme, as the authors (Verma and Altrin 2016, who also supported the idea to include samples of <30% SiO2 during personnel communication) have considered only samples having >30% SiO2 contents under this discrimination scheme.

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