FOLDED REGION

Trunilina V.A., Orlov Yu.S., Roev S.P., Zaitsev A.I.

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

A-type granites are found in different structural-tectonic zones of the Verkhoyansk-Kolyma folded region. They were formed at different stages of its formation. Five granite varieties are recognized here, of which the first two are located in the continental blocks of the Kolyma-Omolon microcontinent, and the rest three are confined to intersections of large faults within the Verkhoyansk continental margin. They are all characterized by high alkalinity, high rare metal and low Ca contents, and a post-orogenic and /or anorogenic setting of their formation.

1) Neoproterozoic (590-550 Ma) alkali-feldspathic granites contain magnesian clinopyroxene and hastingsite with high Na₂O values, high-F magnesian biotite, Cr–bearing titanomagnetite and ilmenite, orthite, Cl- and F–apatites, UO₃- and ThO₂ – rich zircon, and thorite. Restite minerals are ferruginous tschermakite and almandine-grossular of the high-grade metamorphic rocks. The granites are peraluminous, highly ferruginous, with their trend on the magmatic series diagram (after Borodin, 1987) transecting the major evolutionary trends: from trachytic T to alkali basaltic AB. Their high Y, Nb, and LREE contents, negative Eu anomaly, and affinity to the geochemical type of rapakivi granites and ultrametamorphic granites make them comparable to A-type granites of the continental within-plate setting. The parent melt was derived from either the lower crustal substrata previously reworked by deep-seated alkali-basic melts or it resulted from a direct syntexis of the mantle and lower crustal melts.

2) Early-Middle Jurassic (218-166+/-19 Ma) alkali-feldspathic and alkali granites terminate rift-related magmatism which began in the Middle Paleozoic with the emplacement of alkali-ultra-basic melts formed after the upper mantle substrata alkalized by deep fluids (Trunilina et al., 1996). Characteristic minerals of the granites are aegirine, low ferruginous arfvedsonite, ftortoramite, eckermannite, high-F biotite, metamict zircon, columbite-tantalite, chevkinite, orthite, fluorite, sphene, and F-apatite. The rocks are peralkaline, ferruginous, metaluminous and subaluminous.. They are close to rare-metal granites of the alkaline series and agpaitic alkali granites. There is a systematic increase in K, REE and I_o (0.7030 to 0.7441) values in the direction from the alkali-ultrabasic rocks to the granites, which is due to the growing intensity of deep fluid fluxes with time and shifting of the zone of melting into the earth’s crust.

3) Early Cretaceous (129-106 Ma) microcline-albite granites intrude into or occur in the immediate vicinity of the syncollisional granitoides. Their characteristic minerals are phengite-muscovite, siderophyllite, lepidolite, zinnwaldite, protolithionite, topaz, amblygonite, Li-phosphates, spodumene, cassiterite, columbite-tantalite, schorl, zircon, and orthite. Minerals of the host granites, E-morphotype zircon typical of the granulites, and magnesian Na₂O-rich clinopyroxene and amphibole are restitic. The rocks are peraluminous, with low REE and Zr values. They belong to the geochemical type of plumasite granites. In terms of their Y, Nb and Zr ratios they are intermediate between the syncollisional and within-plate granites. High I₀ values (0.71052-0.72877) of the rocks suggest the magma chambers were initiated in the crust. The prevalence among zircon crystals of D-morphotype (Pupin, 1980), typical of the mantle and crustal-mantle derivatives, the presence of Cr-bearing titanomagnetite and ilmenite, the fact that their petrochemical trends crosscut the major evolutionary trends on the magmatic series diagram, high Rb/Sr-SiO₂ values typical of the derivatives of alkali-basic melts (Tischendorf, Palchen 1985), indicate that magma generation resulted either from remelting of syncollisional granites under the influence of deep heat and fluid fluxes or from reworking of the lower crustal substrata of complex composition by these same fluxes but with no formation of syntex melts.

4) Early-Late Cretaceous (86-119 Ma) alkali-feldspathic granites and granosyenites are often associated with fractured bodies of alkali-basic rocks. Characteristic minerals of the granites are sanidine or anorthoclase, magnesian Cr-bearing clinopyroxene, labradorite, V-bearing aegirine (in granosyenites), orthite, sphene, titanomagnetite, fluorite, UO₃- and THO₂ –rich zircon (mainly D morphotype), Cl- and F-apatite, pyrope-almandine with 23-55% py, and complex Si, Ti, and Al oxides. Fayalite relics, restitic tschermakite hornblende and almandine-grossular are present. The rocks are ferruginous, peraluminous, and rich in REE. They belong to the geochemical type of the granites of the alkaline series or the rare-metal granites of the alkaline series, and are compositionally similar to within-plate A-type granites. I₀ values (0.7135-0.7147) suggest magma generation from crustal substrata, while high Rb/Sr ratios are typical of the derivatives of alkali-basaltic magmas. On the magmatic series diagram, they form a trend in composition intersecting the major evolutionary trends. These data as well as close spatial association with the derivatives of alkali-basic magmas and the growing total REE content in the course of evolution of the parent melts, particularly at its final stages, suggest that the mantle and crustal source rocks co-existed and interacted for a long time, over the entire period of functioning of granitoid chambers, and that the parent melts had, possibly, a syntex character.

5) Late Cretaceous (85-94 Ma) alkali granites make up separate plutons or central parts of zoned plutons in which they change into granosyenites and quartz syenites toward the periphery, and are spatially associated with subalkaline or alkaline gabbroids. The rocks contain aegirine (or aegirine –augite), sanidine, orthite, sphene, chevkinite, thorite, spinel, and UO₃ – and ThO₂- rich zircon. In terms of their petro- and geochemical composition, the affinity to the rare-metal granites of the alkaline series, highly differentiated REE trends with a deep Eu minimum, the rocks are classified as typical A-granites. They differ from the other studied granites in a higher K content and affinity to the shoshonite series. On the magmatic series diagram, their data points form a trend parallel to the latitic evolutionary trend. I₀ values (0.7118-0.7129) correspond to crustal origin of magma chambers. The data obtained suggest that the parent melt for these granites had a similar origin to the granites of the previous group, and that the mantle-crust mixing became less intense with the onset of the melt crystallization. A higher K-content of the rocks in comparison with the other A-type granites is due to a specific composition of the deep melt that initiated a lower crust melting.

This work was supported by grant 06-05-96008 from the Russian Foundation for Basic Research.

References

Borodin, L.S., 1987. Petrochemistry of igneous series. Moscow: Nauka, 241p. (in Russian).

Pupin J.P., 1980. Zircon and granite petrology // Contrib. to Miner. and Petrol., V.73, P.207-220.

Trunilina V.A., Parfenov L.M., Layer P.V., Orlov Yu.S. and Zaitsev A.A., 1996. The Middle Paleozoic Tommot massif of alkali gabbroids and syenites of the Verkhoyansk-Kolyma Mesozoides and its tectonic setting // Geologiya i Geofizika, N 4, P.71-82 (in Russian).

Tischendorf G and Palchen W., 1985. Zur klassification von Granitoides //Z. Geol. Wiss. Berlin, Bd.13. Hf.5. S.615-627.