Hybridization in the subvolcanic Jaala-Iitti complex and its petrogenetic relation to rapakivi granites and associated mafic rocks of southeastern Finland

Abstract

SALONSAARI, PEKKA T. 1995. Hybridization in the subvolcanic Jaala-Iitti complex and its petrogenetic relation to rapakivi granites and associated mafic rocks of southeastern Finland. Bulletin of the Geological of Finland 67, Pan lb, 104 pages, 56 figures, 7 tables, and 2 appendices. The 1630 Ma Jaala-Iitti complex is an example of bimodal rapakivi granite magmatism in which the interaction of granite and diabase magmas have led locally to hybridization. The dyke-like complex is situated at the northwestern margin of the Wiborg rapakivi batholith in southeastern Finland, cutting both the Proterozoic Svecofennian metamorphic crust and the Wiborg batholith. The complex consists mainly of non-hybridized compositionally homogeneous granites, i.e., hornblende granite and hornblende-quartz-feldspar porphyry which represent the felsic end-member (ca. 68 wt% Si0 2) of the complex. The mafic end-member (ca. 51 wt% Si0 2) is present as globules of disaggregated Fe-rich tholeiitic magma forming magmatic mafic enclaves (MMEs) and composite MMEs. Commonly MMEs and large (up to 2 metres in diameter) pillow-like MMEs show magma mixing and mingling characteristics. Hybrid rocks in the complex are monzogranitic and are characterized by (a) quartz grains and quartz aggregates (partially melted xenoliths) mantled by amphibole rims (ocellar texture) with occasional augite, (b) alkali feldspar megacrysts mantled by a micrographic plagioclase-quartz intergrowth, and (c) alkali feldspar megacrysts mantled by a plagioclase shell with occasional amphibole inclusions. These textures are also found in hybrid MMEs and especially in pillow-like MMEs. Alkali feldspar and quartz megacrysts in hybrid rocks are derived from partially crystallized felsic magma and from disaggregated rapakivi granite and granitoid xenoliths whereas in the MMEs the xenocrysts are solely derived from partially crystallized felsic magma. Disaggregation of the mafic magma to form MMEs and equilibration with the host have produced micro-enclaves and recrystallized borders of MMEs. Disaggregation of the mafic magma has also produced needle-like apatite crystals incorporated from the mafic magma into the hybrid magma; acicular apatite is typical in MMEs and pillow-like MMEs and is indicative of rapid crystallization. The mass-fraction of mafic magma (X m) in the hybrid rock is up to 0.3. The X m in hybrid MMEs and pillow-like MMEs varies from 0.4 to 0.9. The temperature difference between the two magmas probably caused convection that spread the disaggregated mafic magma throughout the postulated layered magma chamber. The injected mafic magma simultaneously mingled and mixed with the felsic magma which was superheated, assisting the chemical exchange. The interaction of hybrid MME magma with the hybrid magmas was possibly made more effective by diffusion of fluorine from the felsic magma into the mafic magma. The composition of minerals indicates intensive interaction of the mafic magma and the felsic host, with the temperature of equilibrium being in the range of 900° t...

The rapakivi granite plutons of Bodom and Obbnäs, southern Finland: petrography and geochemistry

“…13b): the average (La/Yb) N for the hybrid granitoids is 15.2 and Eu/ Eu* is 0.62. The chondrite-normalized average REE patterns of the rocks of the Obbnäs pluton resemble the patterns of the hybrid rocks (MME) of the Jaala-Iitti complex (Salonsaari 1995).…”

“…The megacrysts of the sparsely porphyritic granite -alkali feldspar, plagioclase, and quartz -are commonly rounded and have a slightly corroded appearance, which is one of the features indicative of mingling of mafic and felsic components in a bimodal magmatic association (e.g. Cantagrel et al 1984, Didier 1987, Salonsaari & Haapala 1994, Salonsaari 1995. Among the other features typical of the hybrid rocks, needle-like apatite is found in the granitoids of Obbnäsudden, but corona textures (amphibole rims around quartz grains and micrographic pla- gioclase-quartz mantles surrounding alkali feldspar grains) are not present.…”

“…The porphyritic hornblende-biotite granite of (Sahama 1945, Lokka 1950, Rieder et al 1996, Suomenniemi (Rämö 1991), Jaala-Iitti (Salonsaari 1995), Ahvenisto (Savolahti 1956), Laitila (Sahama 1945, Lokka 1950, Vorma 1976, Vehmaa (Sahama 1945), Eurajoki (Haapala 1977), and Åland (Sahama 1945).…”

“…13a). The hybrid granitoids (including the magmatic microgranular enclave) are slightly enriched in HREE in comparison to the porphyritic horn- (Rämö 1991), and b) the porphyritic hornblende-biotite granite and the hybrid granitoids of Obbnäs compared to the magmatic mafic enclaves (MME) of the Jaala-Iitti complex (Salonsaari 1995). The normalizing values according to Taylor and McLennan (1985).…”

Mafic-silicic magma interaction in the layered 1.87 Ga Soukkio Complex in Mäntsälä, southern Finland

“…Most of the models proposed to explain how the interaction of different magmas takes place in the plutonic environment involve intrusion of mafic magma into a silicic magma chamber (Blake et al 1965, Vernon 1983, Huppert & Sparks 1988, Lindberg & Eklund 1988, Nironen & Bateman 1989, Pitcher 1991, Michael 1991, Neves & Vauchez 1995, Salonsaari 1995, Wiebe 1993, Hall 1996, Mellqvist 1999, Bergantz 2000, Preston 2001). However, the opposite could also be important and possible (Wiebe 1987a,b, Koyaguchi & Blake 1991, Weinberg 1997.…”