Geochemical and Nd isotopic constraints on the evolution of Neoarchean continental crust underlying the central Deccan Traps

Ancient rocks documenting early silicate Earth processes are only sparsely preserved on its modern surface. Some of the oldest known crustal lithologies (≤3.7 Ga) can be found within the Indian Shield. However, a substantial area of the western and central Indian basement has been covered by the ~66 Ma old Deccan flood basalts. Some Deccan-related mafic dykes in the Nandurbar-Dhule region of the Narmada-Tapi rift zone host xenolithic crustal material, which can be used to study the otherwise inaccessible basement. Textural and mineralogical heterogeneity amongst these xenoliths implies that they derive from different depths of a single column of crust and represent randomly sampled crustal rock types with possibly distinct heritages. Well studied examples of these dykes are the adjacent Rajmane and Talwade dykes south of Duhle, which host Neoarchean-aged [1] crustal xenoliths with highly variable ⁸⁷Sr/⁸⁶Sr ratios between 0.70935 and 0.78479 [2]. This led previous researchers to infer a genetic relationship of these xenoliths with rocks from the Dharwar Craton [1, 2].

In this study, xenolith samples are used to investigate the evolution of sub-Deccan continental crust and evaluate whether randomly sampled crustal lithologies share a common Hadean heritage that is similar to published data for Dharwar granitic rocks. Our samples (n = 17) originate from two mafic dykes near Talwade and Ranala in the Nandurbar-Dhule region. We report major and trace element abundances and ¹⁴²Nd isotopic compositions. The CIPW norms of xenoliths define a nearly continuous petrological evolution trend from tonalites to reworked, orthoclase-rich granites, with subordinate trondhjemitic compositions. The vertical cross-section of crust underlying the dykes therefore provides an opportunity to study the geochemistry of evolving primitive continental crust. Trace element abundance data also conform to a tonalite-trondhjemite-granodiorite-like (TTG) composition for a subset of the xenoliths, whereas others resemble younger granitoids, which might represent reworked TTG equivalents, or younger intrusions.

The short-lived (t_1/2 = 103 Ma) ¹⁴⁶Sm-¹⁴²Nd decay system is particularly sensitive to magmatic fractionation processes that occurred within the first ca. 500 Ma of Earth’s history. Heterogeneous ¹⁴²Nd/¹⁴⁴Nd compositions (expressed as μ¹⁴²Nd = [(¹⁴²Nd/¹⁴⁴Nd)_sample/(¹⁴²Nd/¹⁴⁴Nd)_JNdi – 1] * 10⁶) are typically restricted to Archean-aged rocks and reveal information about the preservation of mantle heterogeneity over geological timescales. The μ¹⁴²Nd of dyke host lavas (n = 3) are heterogeneous (μ¹⁴²Nd = -2.0 ±5.1 to +6.1 ±5.1) but unresolved from the terrestrial standard. Such heterogeneity suggests that the parental magmas to the dykes experienced complex lithospheric and crustal assimilation during their ascent. Felsic xenoliths have homogeneous μ¹⁴²Nd compositions (μ¹⁴²Nd = -0.9 ±2.3, 95% c.i., n = 7). Combined with the major and trace element data, this implies an extensively reworked crust underneath the Deccan Traps. The lack of recognizable μ¹⁴²Nd anomalies is consistent with data of younger Dharwar granitoids [3] and may reflect regional overprinting of mantle μ¹⁴²Nd heterogeneity at or before the Neoarchean emplacement age of the xenoliths.

 

[1] Upadhyay et al. (2015) J. Geol. 123(3), 295–307.

[2] Ray et al. (2008) Gondwana Res. 13, 375–385.

[3] Ravindran et al. (2022) Goldschmidt Abst. 10986.