PETROCHEMICAL PARAMETERS OF ULTRAMETAMORPHIC GRANITOIDS AS

REFLECTING THEIR CRUST AND MANTLE NATURE

Levitsky V.I., Levitsky I.V.

Institute of Geochemistry SB RAS, Irkutsk, Russia, vlevit@igc.irk.ru

The mechanism of anatectic melting of metamorphic substratum or its metasomatic transformations under the action of mantle fluids, i.e. emplacement of granitoids in the deep-seated zones of continental crust, has not been studied well enough so far. Some researchers are prone to stick to the anatectic model, although some of its aspects are not sufficiently justified.

The anatectic model is based on the experimental data on clay melting in closed systems (overview after Menert, 1971). When working in the field we accept the criterion of anatexis to recognize in the substratum (paleosom) of neosom (mobilizates, migmatites, metatectites, diatectites and leucosom) and restites (melanosom). In the experiments on basic rock melting it was feasible to identify origination of only small volumes of plagiogranites. In melting many protoliths granites were truly melted, but the residue contained unusual rare rocks. Granites showed the deficit in К, Li, Rb, Cs, Ba, Sr, Zr, LREE and excess of ferrum-group elements, as compared to natural associations. Besides, in such a way it is not possible to experimentally produce granites from marbles, quartzites, high-alumina schists, meta-hyperbasites.

The idea on formation of granitoids through metasomatic transformations has been known in literature since the end of the XIX age as transformism, granitization and ultrametamorphism. From this model formation of granites is feasible through transformation of rocks of any, even basic compositions with the supply of constituent elements by deep mantle fluids. These views have been confirmed by the experiments only recently for some technical reasons (Khodorevskaya, 2006), but geologically and petrologically they are justified much better than the anatectic model of granite formation. From this point of view it is important to gain geochemical evidence of granitoid formation from rocks of any substratum in the processes of crust-mantle interaction. In the deep crust zones there is the following rock series from substratum to newly formed granites and their analogs: original protolith – plagiomigmatite – kalispar migmatite – shadowy kalispar migmatite. At the same time there are formed basificates, the rocks enriched in the bases.

When studying granite formation (granitization, ultrametamorphism, ultrametagenesis) we identified in the basement of the Siberian platform, Baltic shield and in the Pamirs, that the composition of granites and their analogs depends on: (1) chemical specifics of substratum; (2) composition of affecting mantle fluids; (3) dynamic setting (compression or extension) of proceeding processes; (4) degree of magmatic differentiation and manifestation of acid-basic interaction of melts in the contacts of different environments (Levitsky, 2005). Тhus, charnokites, hyperstene granites and plagioclasite are developed after bipyroxene schists; syenites after meta-hyperbasites; high-alumina schists - leucocratic and subalkaline granites after gneisses; leucogranites after montmineral quartzites; magnetite enderbites after magnetite quartzites; granosyenites, pyroxene, amphibole, nepheline syenites after amohibolites, dolomitic marbles; cordierite and garnet granites, leucogranites after magnesitic rocks; pyroxenitie and amphibolite syenites, svyatonosites after calcitic rocks. The composition of migmatites, developed after different substrata, is given below (Table 1).

The granites, developed after alumosilicate and carbonate rocks, show the contents of Li, Rb, Cs, B, F, Mo, La, Ce, Nd, Yb, Y, Zr are lower, Be, Zn higher, and Rb, Ba, Sr, Sn, Pb, Cu, Cr, V, Ni, Co, Sc at the level of clarkes of acid rocks. The highest abundances of Ва, Rb are observed in syenites occurring in dolomitic marbles. The syenites in ultrabasic rocks have very high contents of Cr, V, Ni, Co and Sc. While granitoids were forming all groups of substratum exhibited an increase of K, Rb, Ba, Pb, Zr, Si, Na, Sr, La, Ce, Nd, Cs, Ta and Nb contents. The fact that they increase in all regions, though at different scale, indicates the deep and mantle origin of fluids.

Table 1. Mineral (average-weighted) characteristics of magmatic rocks developed after bipyroxene schists (1); meta-hyperbasic rocks (2); low- and moderately aluminous gneisses (3); tonalite-trondjemite gneisses (4); high-alumina schists (5); single mineral (6) and ferruginous quartzites (7); dolomitic (8), magnesitic (9) and calcitic (10) marbles.

	1	2	3	4	5	6	7	8	9	10
SiO2	69,50	59,35	74,07	74,29	74,69	74,46	69,27	64,12	73,95	59,67
Al2O3	15,14	14,12	13,97	14,12	13,50	14,25	12,10	18,11	14,75	16,06
Fe2O3	3,26	7,90	1,45	1,24	2,95	1,55	8,75	2,24	1,52	4,10
MgO	0,86	8,22	0,32	0,27	1,04	0,23	0,99	0,82	0,61	0,83
CaO	2,30	5,93	1,25	1,59	1,14	0,39	2,78	3,71	0,97	5,75
K2O	4,65	1,66	4,53	3,86	3,95	5,30	2,10	5,79	3,77	6,57
Na2O	3,41	1,09	3,54	3,99	2,46	3,09	2,85	3,77	3,56	3,90

Table 1. (continued)

	1	2	3	4	5	6	7	8	9	10
Rb	124	77	134	115	127	225	46	176	90	174
Ba	1540	353	915	1323	745	714	537	4187	340	2838
Sr	377	110	224	400	230	154	263	395	72	1464
La	38	25	40	13	27	13	17	41	5	75
Ce	65	32	74	21	59	40	31	78	8	121
Y	17	6	21	3	29	25	6	15	12	49
Zr	165	120	120	87	160	52	132	219	39	103
Pb	23	1	35	27	21	83	12	27	29	17
Cr	34	573	15	8	14	15	54	36	4	15
V	42	135	11	14	28	5	54	34	2	51
Ni	17	228	10	5	13	13	37	32	3	9
Co	8	36	3	2	4	1	9	3	1	6

The mineral features of ultrametamorphic granitoids disclose the mechanism of their crust-mantle origin. One can accept or disregard the idea on the significant role of metasomatism in granite formation, however with any approach applied it is vital to propose new models explaining the influence of both substratum on the composition of magmatites and their enrichment in the spectrum of elements, their mantle source causing no doubt with some researchers.

The work is accomplished with support from RFBR grant 08-05-00322-а.

References

Levitsky V.I. Petrology and geochemistry of metasomatism in continental crust formation. Publ.H.: «Geo» Branch in Novosibirsk, 2005. 343 p.

Menert K. Migmatites and origin of granites. М.: Mir, 1971. 326 p.

Khodorevskaya L.I. Experimental studies of granite formation after the rocks of basic composition. Doctoral Degree Thesis. MSU. 2006. 42 p.