Model Types of Tonalite-Trondhjemite Melts and Their Natural Counterparts

Abstract

Several models have been proposed for the origin of tonalite-trondhjemite (TT). The most likely petrogenetic process is considered to be partial melting of mafic sources such as amphibolite, eclogite and garnet-bearing granulite that is proved by experiments on natural amphibolites. It is clear that trace element composition of TT mainly depends on residue mineral association, which in turn is closely related to P T conditions. Consequently the composition of natural TT can be used to evaluate the melting conditions. Available experimental data (residue mineral content and degree of melting) (Beard and Lofgren, 1991; Sen and Dunn, 1994; Rapp and Watson, 1995; Winther, 1996) allow to calculate possible concentrations of some trace elements in model TT melts generated in wide range of P (3-22 kbar) and T (800-1040~ To cover range of probable natural mafic rocks, trace element contents in model sources are assumed to be equal to that in tholeiites of the Archaean greenstone belts and MORB. Five types of model TT melts with different geochemical characteristics correspond to following distinct restites: gabbroid (Pg + Cpx _ Opx) (1), amphibolitic (Pg + Hb) (2), garnet-amphibolitic (Gar + Pg + Hb + Cpx) with low ( 20%) garnet content (3) and (4) and eclogitic (Gar+Cpx) (5). From first type to fifth the model TT melts are progressively depleted in HREE, Y and enriched LREE and Sr due to change of residue association. Analysis of data set on the Precambrian and Phanerozoic TT suites shows that natural TT are comparable to their model counterparts in REE, Y and Sr contents. Proposed Yb-Eu diagram (Fig. 1) allows to reconstruct the general petrogenetic history of the natural TT. Yb concentration indicates the type of residue association and conforming most likely P T region of melt generation. In addition the trends of simultaneous Eu and Yb depletion allow to evaluate degree of fractional crystallization of parental melt since crystallization of main phases (hornblende and plagioclase) from TT melt decreases these element concentrations. Excepting TT formed in low P (types 1 and 2) in according to Yb and Y content the most of the Archaean TT (not less than 70%) could originate in equilibrium with eclogitic and garnet-enriched amphibolitic restites. Proportion of TT extremely depleted in HREE decreases among the Proterozoic granitoids. Among the Proterozoic plagiogranitoids 4 and 5 types of TT are equally representative. The Phanerozoic TT and adakites have still higher typical Yb concentration (>0.65-0.7 ppm) that make impossible their origin in equilibrium with eclogitic and garnet-enriched amphibolitic restite. Thus variations in Yb show tendency of change of dominant residue associations and consequently prevailing P-T conditions of melting from the Archaean to Cenozoic. Probably adakites can not represent