Geochronology and isotope geochemistry of Eocene dykes intruding the Ladakh Batholith

In order to determine the extent and timing of dyke formation related to possible E-W extension along the southern margin of Eurasia during Early Cenozoic time, we examined ca. 30 mostly andesitic dykes intruding the Ladakh batholith from 10 to 50km west of Leh (NW India). The dykes in the east of the area trend E-NE and those in the west trend N-NW. The difference in orientation is also evident in the petrography and isotopic signatures. The eastern dykes contain corroded quartz xenocrysts and show negative ε0(Nd) and positive ε0(Sr) values, whereas the western dykes do not contain quartz xenocrysts and exhibit positive ε0(Nd) and near-zero ε0(Sr) values. The variability in Sr-Nd isotopes (ε0(Nd)=3.6 to -9.6, ε0(Sr)=0.4 to 143) and the quartz xenocrysts can best be explained by (differing degrees of) crustal assimilation of the parent magma of the dykes. Separated minerals from five dykes were dated by 40Ar-39Ar incremental heating: amphibole ages range between 50 and 54Ma, and one biotite separate dated both by Rb-Sr and 40Ar-39Ar gave an age of 45Ma. One dated pseudotachylyte sample attests to brittle faulting at ca. 54Ma. The combination of structural field evidence with petrographic, isotopic and geochronological analyses demonstrates that the dykes did not form from a single, progressively differentiating magma chamber, despite having formed in the same tectonic setting around the same time, and that processes such as crustal assimilation and magma mixing/mingling, also played a significant role in magma petrogenesis.