Granulometry of xenolithes as the method for formation of intrusions (on example of Archean massifs of Karelian craton)
Institute of Geology, KarRC, RAS, Petrozavodsk, Russia
Grain-size analysis was developed and is widely used in lithology to study the genesis of clastic rocks. However, it can also be used in igneous rock petrology.
The method to study igneous complexes is based on the opportunity to apply Stokes’ law (1) to description of the movement of solid bodies (xenoliths, crystals) in liquid with a high coefficient of viscosity (magma).
The point is that the velocity of movement () of a xenolith in immobile viscous liquid is described by the equation (Cox et al., 1982; Turcott & Schubert, 1985):
where ρx is the density of xenolith matter, ρl is liquid density, r is xenolith radius, μ is liquid viscosity and g is free fall acceleration (9.8 m/s2).
If a cluster of fragments is in a mobile medium, then xenoliths that move at equal velocity will dominate in this flow section. Consequently, the following proportion is valid for them (Slabunov, 1995):
where ρ1 andρ2 are the density deficits of compositions 1 and 2, respectively, r1 and r2 are the radii of xenoliths of compositions 1 and 2, respectively. Equation (2) follows from Stokes’ law, and the observing of the law indicates the density separation of clasts in a dynamic environment.
This method was used (Slabunov, 1995) to study the conditions of formation of a Neoarchaean sanukitoid batholith that covers an area of about 6500 km2 and is located in the northeastern Karelian Craton, North Karelia (Bibikova et al., 1997). The batholith consists of poorly foliated diorites, quartz diorites, granodiorites and sanukitoid-series plagiogranites.
The rocks of the complex contain numerous xenoliths. Metaultrabasic rocks, amphibolites and amphibole, epidote-amphibole-biotite and biotite schists are distinguished, depending on the mineral composition of the xenoliths.
Grain-size analysis of xenoliths from the batholith has shown their polymodal size distribution and an inverse correlation between size and composition and provided an argument in favour of the density separation of clasts in dioritic magma. In addition, the areal distribution of xenoliths was analysed, and magma movement dynamics in the chamber at late stages in the life cycle of magma was assessed.
The grain-size method was employed to study xenoliths in granites from the Onega complex of the Vodlozero terrain, Karelian Craton.
The rocks are typically medium- to coarse-grained, occasionally show a porphyraceous habit and contain relict inclusions (or xenoliths) of older unequally granitized rocks (substrate). The relics are represented by tonalite-trondhjemite-series rocks and amphibolites (Kostin, 1989).
The complex is assumed to have been formed either by magma-metasomatic replacement of the substrate (Kostin, 1989) or by intrusion of granitic magma accompanied by the formation of xenoliths. In the former case, host rocks are expected to disintegrate without subsequent movement and separation. Consequently, country rock fragment size distribution is expected to show a lognormal pattern, as, for example, in melange (Slabunov et al., 2007; Burdukh, 2009). In the latter case, xenolith size distribution is expected to be polymodal.
Inclusions in granite were subjected to grain-size analysis on the south shore of Glubokaya Kara Bay, Lake Onega. In the sections analysed, the inclusions are typically ellipsoid, smoothly angular or angular. As each fragment was only measured in one arbitrary section, sizes were not fully described. However, these parameters are undoubtedly related functionally to the basic characteristics of body sizes and, consequently, can be used to estimate their variations. Two parameters of fragments, length (measured along the longest axis) and width (measured along the shortest axis) were measured. Measurements were made with a measuring tape to an accuracy of 0.5 cm.
The results of grain-size analysis of xenoliths are shown in Table 1 and in Figure 1.
Table 1. Linear sizes and density of xenoliths in granites from the Onega complex.
Fig.1. Xenolith length distribution (n - sampling size).
The data obtained show that the validity of the equation
where ρl ≈ 2.4 g/cm3 is granitic liquid density (Manual…, 1969).
At the same time, the size distribution of xenoliths differs from lognormal distribution and exhibits a bimodal pattern. The first mode is in the 10-15 cm interval and the second mode is in the 20-25 cm range.
The above data indicate density separation of fragments, which, in turn, shows the validity of the second hypothesis of granite complex formation.
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