Extension in the Baikal rift and the depth of basalt magma generation

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It is considered that the opening of the Baikal riftsystem occurred as a result of counterclockwise rotation of the Amur microplate relative to stable SiberianCraton [1]. The pole of rotation is located on thenortheastern termination of the rift system in the areaof the Chara depression [1, 2] (Fig. 1). According togravimetrical data, it was established that the value ofthe extension increases linearly from the pole of rotation towards the South Baikal depression [2]. Therecent data of the seismic profiling across the SouthBaikal depression show that under this depression therise of the Moho boundary is absent. However, in thebottom parts of the Earth’s crust, a highvelocityanomaly, interpreted as a series of deep mafic intrusions, is established [3]. The Crustal extension is compensated by magma intrusion, and not its thinning [3].At the same time, within the largest Baikal depressions, volcanism manifestations [4] do not occur (Fig. 1).Thus, the problem of interrelation of volcanism andrifting still remains open. In this work, the depth offormation of primary basalt magmas under differentvolcanic fields of the Baikal Rift System is determinedon the basis of the chemical composition of volcanicproducts. It is shown that between the depth of magmageneration and the crustal extension value a reversecorrelation exists; i.e., the stronger the extension, thelower the depth that testifies that rifting processes control volcanism. The role of the inflow of deep mantlematter in generation of basalt magmas is also considered.In the northeastern Baikal rift system, there are tworelatively small Late Cenozoic volcanic fields (~100 kmin diameter), the Udokan and the Vitim (Fig. 1). Theformer is situated on the southern slope of the Charadepression; the latter one, in the area of small depressions outside the axial zone of the rift system. In thesouthwestern rift system, the Late Cenozoic volcanismwas manifested on the vast territory (more 450 km indiameter) within the depressions (Tunka and Khubsugul), on their slopes (as an example, on the KhamarDaban Ridge), as well as in areas without visible extension structures (the East Sayan Ridge).The calculation of the pressure of generation of primary magma was carried out using the proceduredescribed in [5] with insufficient modifications, whichinclude the following: (1) the assumption was usedthat the magma source is similar in chemical composition to enriched KLB–1 lherzolite, but not to the intermediate composition between KLB–1 and HK–66lherzolites; (2) the correction for olivine and clinopyroxene fractionation was not made. The first calculation modification was made on account of the comparison of calculated pressure values obtained formantle xenoliths and the host basalts of the Oka highland [6]. The second modification is connected withnonessential change in the calculated values for compositions of real basalts and compositions, corrected forolivine and clinopyroxene fractionation [5]; i.e., it simplifies the calculation procedure. Finally, the equationwas used based on experimental data for melting ofKLB–1 lherzolite [7]: