2013

Geochemical types of basites in the eastern part of the Siberian platform

Kopylova А.G and Tomshin M.D.

Diamond and Precious Metal Geology Institute, SB RAS, Yakutsk, Russia kopylova@diamond.ysn.ru

Basic magmatism repeatedly manifested itself over a long period of time in the Siberian platform, with a changing spectrum of the melt composition. Permian-Triassic magmatic activity in the eastern Siberian platform was connected with the intrusion of enormous volumes of basic magma, which formed rocks of trap formation with normal tholeiite-basaltic specialization. Blanket basalts of this age are almost absent in the eastern part of the platform and there are no intrusions of picrite-basalt composition. Petrochemical variety of magmatites of this formational type is due to processes of pre-chamber and intra-chamber differentiation but of crucial importance in determining their petrochemical and geochemical features are processes of initial magma formation. Slight variations of composition of the studied traps from different parts of the platform (central, eastern and north-eastern), formed by the melt of tholeiite composition, indicate similar conditions of their magma formation. Average contents of SiO₂ in trap formations are within limits of 48-49, TiO₂ - 1,1-1,7, K₂O -  0,4-0,7, P₂O₅ - 0,14-0,18, and total Fe content is 1-13 % (Table). On SiO₂ – (NaO₂+K₂O) diagram, figurative points of composition fall into the field of tholeiites of normal alkalinity. Distribution trends of incompatible elements in traps from different parts of the platform occupy the same position on spider diagrams. (La/Yb)_n ratio varies within narrow limits (2,2-3,5) indicating similar REE distribution in traps. Presence of Ta-Nb-minimum on multi-element diagrams and geochemical indicators of crust contribution (Nb/Nb*=0,63 and La/Nb>1,2) in the trap formation rocks are indicative of crust contamination.

 The Middle Paleozoic eastern periphery of the Siberian platform was noted for vigorous magmatic activity connected with the development of paleorift systems. The biggest of them are the Viluy and Olenek paleorifts. Magmatic activity lasted long within the limits of rift zones, and the accompanying geodynamic setting was unstable, which affected composition of the rocks. Сentral parts of rift valleys were under extension, and the melt rose quickly.  Typical basites of Middle Paleozoic rifts with low content of phenocrysts (up to 3%) in marginal zone rocks were formed. In the classification SiO₂ – alkalies diagram, points of rock compositions of Middle Paleozoic paleorift zones are almost equal in the fields of quartz-normative tholeiites and basites of subalkalic composition.  As compared with the trap formation rocks, Middle Paleozoic basites are characterized by increased Ti, K, P, HSFE, and LILE contents. Total content of rare earths is almost twice as high, and ratio (La/Yb)_n=5,7. Multi-element diagrams of incompatible elements distribution don’t show a Ta-Nb-minimum, which is indicative of no crust contamination of the melts responsible for Middle Paleozoic magmatite formation.

Spatially associated with the trap intrusions are trachydolerite bodies formed on the north-eastern margin of the platform. They are formed by alkali olivine-basalt series of melts and are confined to the Lena-Anabar deep-seated fault. Trachydolerites in the eastern part of the Siberian platform are of Lower Triassic age and they complete the cycle of Phanerozoic magmatism on the Siberian platform. Trachydolerites are characterized by low SiO₂ content (avg. 44 % SiO₂) and high alkalinity (total alkali content 4,5-6 %). Figurative compositional points of trachydolerites fall into the field of alkali series.  Significantly, trachydolerites are high in titanium (6,4 % TiO₂), iron (>14 % FeO_tot.), and phosphorus (up to 1,4 % P₂O₅). High content of titanium group elements (Zr, Nb, Y, Yb, Hf, Ta) is the most prominent geochemical feature of trachydolerites. Distinct enrichment of the derivatives of alkali olivine-basalt melt in large ion elements– Cs, Ba, Rb, Sr, Pb, and K indicates a great effect of the fluid phase. Trachydolerites show maximum total content of REE with a highly fractionated distribution (ratio (La/Yb)_n = 24,9). Rare element features of trachydolerites indicate that lithospheric material was not involved in magma formation. They are much lower in compatible elements Ni, Co, Cr, and Sc than derivatives of tholeiitic melts. It should be noted that trachydolerites contain increased concentrations of such ore components as Pb, Be, Mo, Sn, Ga, and Gd, which are incompatible with basic magma.

            Thus, different-aged intrusive formations of the eastern Siberian platform are, geochemically, classified into Late Paleozoic – Early Mesozoic platform magmatities of trap syneclises, formed by tholeiite-basalt series of the melts, Middle Paleozoic high-alkalinity basites of paleorift zones of tholeiitic composition, and Early Mesozoic trachydolerites  of peripheral parts of the platform, which are the derivatives of alkali-olivine basalt magma.

Table. Average compositions of intrusive rocks in the eastern part of the Siberian platform (wt % and ppm)

Note: 1 and 4 – Permian-Triassic traps, 2 and 5 – Basites of Middle Paleozoic rifts,

3 and 6  - trachydolerites . N – number of analyses.