Résumé
The Kohistan–Ladakh Terrane in the NW Himalaya is a remnant of a Cretaceous arc sequence obducted onto the Indian margin. This paper presents a geochemical study (major and trace elements and Sr, Nd, Pb isotopes) of the Mid-Cretaceous lavas of the Ladakh side of the arc sequence, which were erupted in response to northward subduction of Neo-Tethys oceanic crust.
Lavas from the western Ladakh in Pakistan can be divided into three groups which, from north to south, are: (1) the Northern Group of back-arc tholeiites [0.5<(La/Yb)
N<1.4; 0.3<(Nb/La)
N<1.4; 4<εNd<8; 38.66<
208Pb/
204Pb<38.80], (2) the Southern Group of arc tholeiites [1.8<(La/Yb)
N<3.9; 0.1<(Nb/La)
N<0.6; 5<εNd<6; 38.40<
208Pb/
204Pb<38.66], and (3) the Katzarah Formation of tholeiitic Nb-rich lavas [3.4<(La/Yb)
N<9.8; 1.4<(Nb/La)
N<2.1; 3<εNd<5], including radiogenic Pb lavas [39.31<
208Pb/
204Pb<39.51] and less radiogenic lavas [38.31<
208Pb/
204Pb<38.55]. Magmas from the eastern Ladakh in India show a simple series of more evolved arc volcanics from basalts to rhyolites [basalts and basaltic andesites: 2.5<(La/Yb)
N<5.7; 0.4<(Nb/La)
N<0.5; 1.8<εNd<5.5; 38.70<
208Pb/
204Pb<38.80]. Isotope and trace element data of western Ladakh lavas are compatible with high-degree melting (14–21%) of a fertile MORB-mantle source. An adakitic lava [(La/Yb)
N=55.8; (Nb/La)
N=0.3; εNd=1.7;
208Pb/
204Pb=39.00] and a Mg-poor Nb-rich basalt [(La/Yb)
N=4.6; (Nb/La)
N=1.3; εNd=−2;
208Pb/
204Pb=39.07] are spatially associated with the tholeiitic arc lavas. Isotope compositions of all the lavas, and in particular the radiogenic Nb-rich and adakitic lavas suggest three-component mixing between depleted mantle similar to the Indian MORB mantle, and enriched components similar to the volcanogenic or pelagic sediments. The geochemical diversity of magma types is attributed to contribution of melts from the subducted crust and associated sediments, and their subsequent interaction with the mantle. Such melt–mantle interactions can also be inferred from relicts of sub-arc mantle found in Indian Ladakh. These results lead to a geodynamic reconstruction of the Kohistan–Ladakh arc as a single entity in the Mid-Cretaceous, emplaced south of the Asian margin. Slab melting imply subduction of young oceanic crust, as already proposed for the Oman ophiolite farther west. The fast northward drift of the Indian Plate could have triggered wide-scale inversion of the divergent tectonic regime responsible for the opening of the Neo-Tethys Ocean. Our results suggest breaking of the young oceanic crust initiated at the ridge rather than at passive plate boundaries.