Geological Structure of Stromatic Migmatites - Term Paper Example

Introduction A migmatite is a mixture of metamorphic and igneous rock. It is formed when rock such as gneiss i.e. metamorphic rocks melt and crystallize into an igneous rock hence becoming o mixture of the unmelted metamorphic rock and the recrystallized rock known as diatexites. Therefore, a stromatic migmatite normally comprises of a green gray-pink gray gneiss of width range 2–5 cm thick and locally rimmed by thin and dark mineral bands that constitute most of its volume and that of the outcrops. (Atherton & Gribble, 2003) Both the petrographic and geochemical evidence suggests that the stromatic coarse-grained rocks are formed by most of the fluid-present accompanied by equilibrium melting of the dioritic gneiss and by a crystallization dominated feldspar. Partial melting in the middle to lower crust may take place in response to dehydration of hydrous minerals such as muscovite, biotite, and amphibole or influx of externally derived hydrous fluids. (Mehnert, 1998) Geochemical evidence suggests that the melts may undergo a process leading to formation of various indigenous rocks, either in situ or while migrating from the site of melting hence migmatites will form in regions of high structural forces, thickened crust and a number of field and experimental studies have indicated a significant potential for melt migration during deformation.  Despite several recent studies regarding the structure and formation of stromatic migmatites, a number of questions regarding the origin of these rocks remain unanswered. These unresolved issues include the role of fluid during partial melting i.e. whether the abundant coarse-grained rocks represent in situ or externally derived melts, the extent to which coarse-grained rocks compositions were modified by fractionation, and the possible relationships between melting, melt migration and the forces due to their origin. (Raymond, 2002) The purpose of this paper is to present an analysis of data from both the origin and chemical composition in an attempt to explain the geological structure of stromatic migmatites and thereby contribute to a better understanding of the causes and consequences of partial melting and other large collisional forces resulting in the formation of these structures. Stromatic migmatites The two types of migmatites i.e. stromatic and patch migmatite are distinguished based on the morphology of the constituent grains and type of host rock. Stromatic migmatites are typically light grey, quartz monzodioritic to greenish gray- pinkish gray coarse-grained rocks while patch migmatites have dioritic mesosomes grains, and are typically found as outcrop-scale bodies within the stromatic migmatites. (Ashworth, 1995) Because of their generally lower strain, these rocks provide some constraints on melt generation and transport processes indeterminable from the stromatic migmatites. Some migmatites preserve their initial structural compositional banding and The layering and can be classified as metatexite. (Winkler, 1999) Characteristics of Stromatic migmatites Stromatic migmatites are characterized by a layered structure made up of 0·5–5 cm thick layers ranging from medium- grained to coarse. They also constitute strongly recrystallized granitic coarse-grained rocks which are normally a few meters long. The host rock is commonly banded with centimeter-scale layers of dark grey grains. (Ashworth, 1995) This illustrated in Figure 1 below. The coarse-grained rocks are generally rimmed by thin, discontinuous dark mineral bands, referred to as concordant dark mineral bands. For the scope of this paper, only concordant to slightly structurally unconformable coarse-grained rocks are sampled for geochemistry and for convenience, these coarse-grained rocks are referred to as structurally conforming. Figure 1: Concordant coarse-grained rocks in stromatic migmatites (Source: Ashworth, 1995) Figure 1 above illustrates concordant coarse-grained rocks in stromatic migmatite. In (a), the arrows indicate locations where a structurally unconformable relationship is observed between different generations of coarse-grained rocks. In (b), it shows a small coarse-grained rocks patch in boudin neck. In (c), it shows part of outcrop with a very large proportion of coarse-grained rocks, containing small, rotated blocks of ‘normal’ stromatic migmatite while in (d), it shows structurally unconformable coarse-grained rocks in small shear bands. The coarse-grained rocks that conform to geological structures constitute about 20–30 % of the stromatic migmatites and are measured through the outcrops on a series of vertical cross-sections. However, the amount of these rocks varies strongly within an outcrop i.e. the stromatic migmatites contain rotated blocks of migmatitic gneiss floating in coarse-grained rocks where the proportions reach 50% volume as illustrated in Figure 1(c) above. The concordant coarse-grained rocks are variably folded and, in places, younger coarse-grained rocks can be seen cutting older, deformed coarse-grained rocks (Figure 1a). The 20–50 % volume of coarse-grained rocks is therefore cumulative, and is unlikely to represent the amount of melt present at any one time, nor does it represent the fraction of melt generated by melting of the remnants of the original rock. Normally, the melts appear to have migrated along The layering planes and accumulated within shear bands or in boudin necks (Figure 1b). These observations suggest that melting and its movement in the rocks have same age and origin with deformation. (Ashworth, 1995) Normally, stromatic migmatite have melt patches in their system and veins which have been derived locally. They are normally folded intensely as illustrated in Figure 2A and have a crenulation cleavage at their axial planar structure. The layering is variable due the folding but fold axial surfaces are generally dipping to the south. The stretching lineation is parallel to the fold axes and generally has a shallow or intermediate plunge to the east. (Gast, Alleì€Gre & Hart, 1998) The folds appear to be fairly cylindrical which is probably due to strong stretching due to shear. The folds are most likely the obvious folds on the magnetic maps. All rocks but the red granites are strongly deformed and re-structured. However, the red granite is also layered sometimes with two layers. Even though, it indicates the minimum age for the folded and refolded stromatic migmatites and related diatexites that are replaced by the granites (Fig. 2C), and the maximum age for the development of layers in the dykes. (Gast, Alleì€Gre & Hart, 1998) Figure 2: A refolded stromatic migmatite at the coast. B) Oblique the layering in a red granitic dyke. C) A red granitic dyke truncating stromatic (Source: (Gast, Alleì€Gre & Hart, 1998) The very high coarse-grained rocks proportions characteristic of stromatic migmatites are inconsistent with in situ partial melting, because greenish gray- pinkish gray rocks are unlikely to yield such high melt proportions and because complementary residual rocks are missing. (Winkler, 1999).  Although the concordant dark mineral bands may represent residues after partial melting, the small volume of dark mineral bands which are typically only 1–2 mm thick cannot account for the volume of coarse-grained rocks in the rocks. A rigorous mass balance calculation can be prevented by difficulties in estimating the composition of the structurally conforming dark mineral bands and the interpretation that most of the coarse-grained rocks do not represent melt compositions. (Arth, 1996) Therefore, the geometry in stromatic migmatites could result from difference in the timing of coarse-grained rocks formation i.e. the migmatites are formed post-kinematically and from in strain during deformation. Geochronological data relating to the formation of stromatic migmatites indicate that the concordant coarse-grained rocks are crystallized and that high-grade structural changing and deformation is persistent hence making it possible that the stromatic migmatites formed later in the metamorphic history and therefore experienced less deformation. Although the exact mechanism by which stromatic structures develop is obscure, Mehnert, (1998) suggests that they can develop in response to applied differential stress acting on a strongly direction dependent original rock such as the banded, granodioritic host rock to the stromatic migmatites. In contrast, the host rock to the stromatic migmatites is stronger and more homogeneous, hence may have possibly resulted in a stress field that was closer to all directions hence their patch geometry. Stromatic migmatite rocks are fairly well-preserved amphibolite faces supracrustal rocks, granitoids and mafic metavolcanic and plutonic rocks. There is evidence of at least two fold phases in the migmatites and are shortened by very crystalline granites. The boundaries of the shortened red granites are very sharp. When the red granites locally form larger bodies, they have well preserved magmatic texture showing that the rock has not been strongly deformed. (Atherton & Gribble, 2003) However, in granitic dykes there is locally a boundary parallel layering and often an additional oblique layering. A fairly distinct layers appears to be parallel to the axial surfaces of the folds in the migmatites. Thus, there must have been some post-dyke shortening after the folding occur to explain the shared layering in the dykes and the migmatites. The boundary parallel layer in the dyke is defined by bands of feldspar, and the oblique layers by a grain shape fabric of quartz. These layers define an S/C-fabric indicating dextral shear in and along the boundaries of the dykes and in turn that the dykes have been rotated anticlockwise. The relationship between the steep axial surfaces of the folds and the layered dykes (Fig. 3B) suggests that N-S shortening bisecting the structures caused the anticlockwise rotation and resulting shear of the dykes and clockwise rotation and flattening of the folds. (Raymond, 2002) Figure 3 (Source: Raymond, 2002) Discussion 1. Origin and Geochemistry of Stromatic migmatites According to the study by Ashworth (1995), the analytic procedures involved in evaluating the origin and geochemistry of stromatic migmatites involve major and trace elements which are determined on fused glass beads and pressed powder pellets, respectively, using standard X-ray fluorescence. Also, rare earth elements (REE) are determined by inductively coupled plasma-mass spectrometry using a Na2O2 sintering technique. The geochemical data of studies are presented in Figure 2 below modes of representative samples are presented. The sampling of the both the structurally conforming coarse-grained rocks is normally done by cutting through the large slabs. However, the dark mineral bands are too thin to be sampled during the geochemical analysis. Melting is confirmed by the minor melt films accompanied by boundaries of grain and patches of feldspar. However, the volumes of melt found in mesosomes cannot be confirmed since there is neither original data nor geochemical evidence. Therefore the less than 0.5 % suggested by Arth, (1996) is volumetrically insignificant to support the characteristic. Figure 4 (Source: Winkler, 1999) The dark mineral bands are generally less than 5–10 mm thick and comprise hornblende, plagioclase, and biotite with accessory allanite, apatite, zircon and titanite. Much of the hornblende in the dark mineral bands is texturally similar to hornblende in the granodioritic mesosome. However, a significant portion is coarser grained with rounded inclusions of quartz that is similar to the patch dark mineral bands. Biotite in dark mineral bands is compositionally similar to biotite in the granodioritic mesosome, but is coarser and forms clusters undergrown and apparently in equilibrium with hornblende. The granodioritic mesosome ranges from 65 to 68 % SiO2. Figure 3 below shows Chondrite-normalized rare earth elements patterns for the stromatic migmatites comparing the various migmatite components, and bivariate trace element diagrams. (Mehnert, 1998) Figure 5 (Source: Mehnert, 1998) 2. Petrogenesis of the stromatic migmatites Despite the high strain, the stromatic migmatites keep both geochemical and petrographic characteristics hence indicating that they evolved in the same way as the patch migmatites that are less formed. In particular, the concordant coarse-grained rocks are rimmed by thin, discontinuous dark mineral bands with textures resembling those observed in the patch dark mineral bands. From the data presented in Figure 2 above, the observations are consistent with fluid-present melting of quartz leaving a hornblende and plagioclase-dominated residue and producing a granitic melt. The REE patterns of the concordant coarse-grained rocks display a more positive Eu anomaly with decreasing total REE, similar to the patch coarse-grained rocks, indicating that feldspar-dominated crystallization was important in their evolution. In general, the concordant coarse-grained rocks have less positive Eu anomalies than the patch coarse-grained rocks. (Atherton & Gribble, 2003) This difference could be due to the fact that the coarse-grained rocks, has a more negative Eu anomaly than the dioritic mesosome, which would result in a primary melt with a more negative Eu anomaly, assuming a similar melting reaction. (Ashworth, 1995) Thus, despite their less positive Eu anomalies, we interpret the concordant coarse-grained rocks to represent crystal-rich melts or cumulates with some trapped, evolved melt; that is, similar to the patch coarse-grained rocks. This interpretation suggests that melts were continuously drained from the rocks, and may explain why the rocks did not develop into diatexites despite having present-day integrated coarse-grained rocks proportions of up to 50 % volume. (Gast, Alleì€Gre & Hart, 1998) The field origin and geochemical evidence from the stromatic migmatites suggests that in addition to in situ partial melting, a significant proportion of the coarse-grained rocks present in the investigated outcrops is derived from external sources. There is no evidence that the coarse-grained rocks reported according to Figure 1 or poor grey gneisses throughout the domain are residual after large-scale melt extraction. This suggests that any externally derived melts must have migrated from structurally deeper and more interior parts of the structural formation than those now exposed in the domain hence implying minimum melt transport distances of several kilometers. (Ashworth, 1995) Several mechanisms are responsible for the melt transfer in the crust and these include, diapirism, dyking, fracture propagation and migration through faults or shear zones. The driving force for melt transfer can be interpreted to be fluid-pressure-driven dilatancy resulting from hydraulic fracturing, which forms a pervasive interconnected network through which the melts flowed. In these rocks fracturing may have been parallel to the layer, controlled by the magnitude of difference in tensile strength normal and parallel to the layers, the orientation of the principal stress axes relative to the metamorphological layers and the differential stress applied to the rocks. However, local structures and coarse-grained rocks-filled veins in the stromatic migmatites, constitute of steep fractures formed, possibly in structurally favorable sites or late in the deformation history. (Raymond, 2002) Field observations from the stromatic migmatites suggest that melt is present during deformation, consistent with geochronological data showing coarse-grained rocks and melt migration in most regions driven by differential pressure associated with syn-melt deformation on the local to regional scale. Melt transfer may have been dominantly vertical towards the upper crust, with originally steep melt-transfer networks subsequently deformed into their present orientations, or melts may have migrated laterally along the sub-horizontal coarse-grained rocks networks observed today. (Atherton & Gribble, 2003) The prevalence of structures with low to moderate dips, even in relatively low-strain regions, is more consistent with the latter possibility. This implies lateral melt migration, and thus lateral pressure gradients, on a regional scale. Therefore, the remnants of a regional-scale, melt-transfer zone developed at the mid-crustal level of a structural force large enough to have developed a significant differential pressure between its core and its flanks. (Gast, Alleì€Gre & Hart, 1998) Conclusion The distinct type of migmatite is stromatic migmatite and this classification is based on coarse-grained rocks geometry and host-rock characteristics. The geometry of coarse-grained rocks in the stromatic migmatites wrap around smaller and less deformed bodies of patch migmatite hence suggesting that the dioritic rocks hosting the stromatic migmatites are stronger than the surrounding granodioritic hypothesis, shielding the coarse-grained rocks from the effects of deformation. Stromatic migmatites are probably formed by fluid-present melting of plagioclase, biotite, and quartz, producing a hornblende + plagioclase-dominated residue and granitic melt. The crystallization of feldspar and quartz produces granitic cumulates, represented by most of the studied coarse-grained rocks. Field and geochemical data from the stromatic migmatites suggest significant melt contributions from external sources may have acted as a mid-crustal melt transfer zone during the contraction deformation. Bibliography 1. Arth, J. G. (1996). Behavior of trace elements during magmatic processes—a summary of theoretical models and their applications. Journal of Research of the US Geological Survey 4, 41–47 2. Ashworth, J. R. (1995). Migmatites. Glasgow, Blackie. 3. Atherton, M. P., & Gribble, C. D. (2003). Migmatites, melting and metamorphism: proceedings of the Geochemical [i.e. Geochemistry] Group of the Mineralogical Society. Nantwich, Shiva. 4. Gast, P. W., Alleì€Gre, C. J., & Hart, S. R. (1998). Trace elements in igneous petrology: a volume in memory of Paul W. Gast. Amsterdam, Elsevier Scientific Pub. Co. 5. Mehnert, K. R. (1998). Migmatites and the origin of granitic rocks. Amsterdam, Elsevier Pub. Co. 6. Raymond, L. A. (2002). Petrology: the study of igneous, sedimentary, and metamorphic rocks. Boston, McGraw-Hill. 7. Winkler, H. G. F. (1999). Petrogenesis of metamorphic rocks. New York, Springer-Verlag. Read More