COMPOSITIONAL VARIATIONS OF GARNETS IN ECLOGITIZED ROCKS, KOKCHETAV MASSIF, KAZAKHSTAN: IMPLICATIONS TO THE ORIGIN AND CRUSTAL PROCESSES

Abstract

Mineral chemistry is widely used in the past three decades to understand original setting and the source composition of rocks, thereby unravelling crustal processes. A good example is chrome spinel. The trace element composition of garnet is also in use to study the composition of protolith in metamorphic rocks since early 2000. The objective of this thesis is to see the plausibility of using garnet major element chemistry to understand about the protolith composition and the crustal processes. The samples used in this study were collected from Kokchetav massif, located around the city of Kokchetav, northern Kazakhstan. This region widely studied and is known worldwide for subduction related high-pressure metamorphism happened ca. 500 Ma ago, during the formation of the Central Asian Orogenic Belt. The Kokchetav massif is also famous for the occurrence of diamond bearing meta-sedimentary rocks. Several garnet bearing rocks samples were collected from the region that includes eclogite, gabbro, garnet amphibolite and pelitic schist along with non-garnetiferous granite, amphibolite and granitic gneiss. Thin- sections of all 15 samples collected were made for petrography; out of which garnet bearing samples were selected for detailed study and analyses. The petrography revealed eclogite has a mineral assemblage - Grt+Omph+Rt+Qtz±Amph; gabbro- Grt+Omph+Amph+Qtz±Plg; garnet amphibolite– Grt+Amph+Qtz with minor inclusions of omphacite and pelitic schist- Grt+Phe+Qz; exhibiting granoblastic-ameboid granoblastic to porphyroblastic textures. The major element chemistry of garnet were determined using SEM-EDS technique. The mineral chemistry shows similar composition for Si and Al composition but variations in Mg, Ca and the Fe contents. Compositionally all garnets are of almandine variety. The XFe= Fe/(Fe+Mg) values of eclogite, gabbro and garnet amphibolite are in the range 0.654-0.799; 0.656-0.665; 0.817-0.865 respectively. However, for the pelitic schist XFe falls between 0.876-0.922, indicating a sharp change in the composition. Since all of the rocks included in this study have experienced regional high-pressure metamorphism, in can be postulated that they are all affected by similar physical conditions i.e., pressure, temperature, although, depth of metamorphism can affect the mineral composition due to changes in geothermal gradient. The thesis argue that a drastic change of Fe content in these rocks esp. the pelitic schist is a reflection of the protolith composition. While, eclogite, gabbro and the garnet amphibolite probably can have common protolith-i.e., Fe rich basalt indicating a fractionated source with crustal inputs; the high Fe content in the pelitic schist shows it has more crustal input, also because of the presence of K, Al bearing phengite mica in its assemblage. Crustal inputs in various levels points towards a mixed source that can occur due to crust-mantle interaction. The most efficient processes where crust interacts with mantle is the due to subduction, as also reported in several previous studies from this region. Hence, the results from this study is a proof that major element composition of garnet can be a good proxy to understand stand petrogenesis and related crustal processes esp.in the metamorphic rocks.