The Siberian lithosphere traverse: mantle terranes and the assembly of theSiberian Craton

The kimberlite fields scattered across the NE part of the Siberian Craton h ave been used to map the subcontinental lithospheric mantle (SCLM), as it e xisted during Devonian to Late Jurassic time, along a 1000-km traverse NE-S W across the Archean Magan and Anabar provinces and into the Proterozoic Ol enek Province. 4100 garnets and 260 chromites from 65 kimberlites have been analysed by electron probe (major elements) and proton microprobe (trace e lements). These data, and radiometric ages on the kimberlites, have been us ed to estimate the position of the local (paleo)geotherm and the thickness of the lithosphere, and to map the detailed distribution of specific rock t ypes and mantle processes in space and time. A low geotherm, corresponding approximately to the 35 mW/m(2) conductive model of Pollack and Chapman [Te ctonophysics 38, 279-296, 1977], characterised the Devonian lithosphere ben eath the Magan and Anabar crustal provinces. The Devonian geotherm beneath the northern part of the area was higher, rising to near a 40 mW/m2 conduct ive model. Areas intruded by Mesozoic kimberlites are generally characteris ed by this higher but still 'cratonic' geotherm. Lithosphere thickness at t he time of kimberlite intrusion varied from ca. 190 to ca. 240 km beneath t he Archean Magan and Anabar provinces, but was less (150-180 km) beneath th e Proterozoic Olenek Province already in Devonian time. Thinner Devonian li thosphere (140 km) in parts of this area may be related to Riphean rifting. Near the northern end of the traverse, differences in geotherm, lithospher e thickness and composition between the Devonian Toluopka area and the near by Mesozoic kimberlite fields suggest thinning of the lithosphere by ca. 50 -60 km, related to Devonian rifting and Triassic magmatism, A major conclus ion of this study is that the crustal terrane boundaries defined by geologi cal mapping and geophysical data (extended from outcrops in the Anabar Shie ld) represent major lithospheric sutures, which continue through the upper mantle and juxtapose lithospheric domains that differ significantly in comp osition and rock-type distribution between 100 and 250 km depth. The presen ce of significant proportions of harzburgitic and depleted lherzolitic garn ets beneath the Magan and Anabar provinces is concordant with their Archean surface geology. The lack of harzburgitic garnets, and the chemistry of th e lherzolitic garnets, beneath most of the other fields are consistent with the Proterozoic surface rocks. Mantle sections for different terranes with in the Archean portion of the craton show pronounced differences in bulk co mposition, rock-type distribution, metasomatic overprint and lithospheric t hickness. These observations suggest that individual crustal terranes, of b oth Archean and Proterozoic age, had developed their own lithospheric roots , and that these differences were preserved during the Proterozoic assembly of the craton. Data from kimberlite fields near the main Archean-Proterozo ic suture (the Billyakh Sheer Zone) suggest that reworking and mixing of Ar chean and Proterozoic mantle was limited to a zone less than 100 km wide. ( C) 1999 Elsevier Science B.V. All rights reserved.