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Petrographic and Geochemical Characteristics of Sn (Nb, Ta)
Cu bearing granitoids (pegmatite, granite) of the Kibara belt (1600-900 Ma) in Maniema province/D.R. Congo

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Authors

도스도스 마쿠투 쿠마켈레

Advisor
이인성
Major
자연과학대학 지구환경과학부
Issue Date
2019-02
Publisher
서울대학교 대학원
Description
학위논문 (석사)-- 서울대학교 대학원 : 자연과학대학 지구환경과학부, 2019. 2. 이인성.
Abstract
콩고민주공화국의 마니에마 지역에 위치한 키바라 벨트의 화강암류 암석(화강암, 거정질 화강암)은 중기 시생대 시기(1600 – 900 Ma)에 관입하였다. 화강암류의 주요 원소 성분을 분석한 결과 SiO2 함량이 71-87%로 규장질 암석에 속하며, 알루미늄 포화 지수(Aluminum Saturation Index)가 1.3~3.1 로 S-type 화강암에 속하는 것을 알 수 있었다. K2O, Na2O, Ga/Al 비, Nb 및 Zr 등의 함량을 도시해 본 결과 역시 같은 결론을 지지하고 있다. 따라서 이 지역의 화강암류는 키바라 조산대의 충돌에 의해 지각 물질의 부분 용융에 의해 생성된 것으로 여겨진다. 또한 여러 주원소 및 미량원소 분석 결과는 충돌대 환경에서 이 암석들이 형성되었음을 지시한다. 미세 단층 및 습곡 등의 지구조 흔적들은 키바라 지구조 운동의 강도를 설명해 주는 증거들을 제시해 주고 있다. 희토류 분석 결과 음의 Eu 이상치가 관찰되며, 경희토류의 분화가 장석에 의해 조절된 반면에 중희토류는 휘석과 티탄철석에 의해 조절된 것으로 여겨진다. 연구지역 화강암류는 coloration index 에 의하면 우백질화강암류로 분류 가능하며, 현미경 관찰 결과와 CIPW norm 결과 우백질의 복운모 몬조그래니트, 복운모 석영질 화강암, 알칼리 장석 화강암 및 쿼졸라이트(quartzolite) 등으로 구분된다. 결정의 크기와 조직으로 구분하였을 때 거정질 화강암, 반화강암 및 화강암으로 나뉘며, 현정질 및 오피틱 조직이 관찰된다. 이러한 화강암류 암석 이외에 석영맥, 편암 및 편마암이 산출된다. 석영맥과 거정질 화강암 내에 주석석이 산출되는 반면에 화강암 내에서는 공작석, 황동석, 반동석, 섬아연석, 코벨라이트 및 휘동석과 같은 구리 광석 광물들이 산출된다. 황동석과 반동석이 주로 공작석과 휘동석으로 대체되는 양상이 나타나며, 휘동석은 다시 코벨라이트에 의해 대체된다. 이러한 화강암질 암석은 변질작용을 받았기에 변질 받지 않은 암석에 비해 SiO2 함량이 약간 증가하는 양상을 보인다. 이러한 화강암류 암석과 광석광물 간의 성인적 연관성이 명확하지는 않다. 그러나 화강암류의 기원은 충돌대 환경과 연관된 알루미늄이 풍부한 퇴적물로 구성된 지각 물질이며, 키바라 충돌대에서 고온 고압의 조건에서 부분용융에 의해 만들어진 규장질 마그마인 것으로 생각된다.
Granitoids (granite, pegmatite) of the Kibara belt have intruded Mesoproterozoic formations (1600-900 Ma) in the Maniema province of Democratic Republic of Congo.
Geochemical results have demonstrated through major elements, for most of these granitoids that SiO2 (71 ̴87 % >65%) is ranging felsic rocks, and their Aluminum saturation Index (ASI=1.3-3.1) which is beyond 1.0, therefore per aluminous rocks and the same index is beyond 1.1 for the majority of these granitoids, therefore related to Stype granitoids
leucocratic (Color index =1.9-17.8 %< 40 %), Ferroan( FeOt/ FeOt +MgO = 0.90-0.99), with high differentiation index(82-98 %) while #mg value is low ( Mg/Mg+Fe = 0.01-0.30), calc-lkaline, medium to high K, Estimated liquidus temperature (597-811˚C) is attributable to felsic rocks(excepted for DMNAKE-1 which is low around 327 ˚C), moderate water contents( estimated H2O=3.7-7.7%)
Density ranging felsic magma( estimated density=2.6-2.8) with a viscosity estimated between1.0-3.1 Pa-s. Kibara belt granitoids display variable span values of incompatibles elements notably Large Ion Lithophile Elements (LILE) (Sr = 2 -104ppm, moderate Rb = 383 - 1000ppm, Ba = 10 - 2408ppm, Cs =21-53ppm) and High Field Strength Elements(HFSE)(Nb=4-88ppm, Ta=0.5-48.0ppm, Hf=1.0-1.8ppm, Zr=20-82ppm, Th=3.0-15.2ppm, U=2.2-20.7ppm, W=1-113ppm, Nb/Ta=0.5-8.0, Sn/Nb+Ta=0.3-8.5)
Moreover, Mantle Rock Forming Elements(MRFE) (Cr≈20, moderate to high Cu=10-4300ppm, high Zn=30-740ppm, low and similar Co≈1ppm, low Ni≈20 ppm) .
The tectonic environment in which the Kibara belt granitoids have evolved, was forged in collisional zone here major and trace elements diagrams notably K2O versus Na2O, (10,000) * Ga/Al versus Nb and (10,000) Ga/Al versus Zr, Rb/30-Hf-3*Ta, Nb-Y-3*Ga plots came up with these conclusions. From that perspective, they are originated from the partial melting of crustal material during the collisional event of the Kibara orogen. This idea is also highlighted other discrimination diagrams notably Y versus Nb, Zr versus Ta/Zr, (Y+Nb) versus Rb, ternary Hf-3*Ta-Rb/30 and multicationic R1(=4Si+11(Na+K)- 2(Fe+Ti)) versus R2(=6Ca+2Mg+Al) leading to collisional environment.
Likewise, some tectonic markers (micro faults, microfolds) have been screened and provide some evidences of intensity and extents of the Kibara tectonic events wielded in a compressive regime(collision).
REE fractionations are strongly caused by some minerals phases notably plagioclases, pyroxenes, amphibole and ilmenite whereby LREE fractionations were controlled by feldspar and most of REE features are displaying negative Eu anomaly where one group shows a soft negative anomaly that we can tie up with feldspar low
content(Eu/Eu*N<<1), ( This the case of DMBAR-1, DMBALE-1, DMKAILO-1, DMKAMA-1) whereas the other group spreading out a strong Eu anomaly can be connected to their highly feldspar content (Eu/Eu*N<1), (Case of DMMOKA-2-1,DMMOKA-2-5 ), conversely HREE were also sorely fractionated by pyroxenes, amphiboles and ilmenite (Case of granitoids from Mokama).
After skimming results of minerals observations under microscope and considering the CIPW norms, it should be noticed that some of them are two mica leucogranites(monzogranite) (DMBAR-1, DMMOKA2-1,
DMMOKA-2-5), some others are two mica quartz-rich granitoids (DMKAILO-1, DMMOKA-2-6, DMMOKA-2-8, DMMOKA-2-9), some are Alkali-feldspar granite (DMBALE-1, DMMOKA-2-7) and quartzolite (DMNAK-1). Accounting of their grain size and texture, we can pick them out into pegmatites, aplites and granites exhibiting mainly phaneritic and ophitic textures (plagioclase in pyroxene and amphibole, muscovite in quartz). In addition to these granitoids, quartz veins and other rocks notably schist and gneiss have been identified. Pegmatite and quartz veins are mineralized in Sn (cassiterite and suites) meanwhile granites are mineralized in Sn as well as in Cu ore which is a breakthrough in terms of mineralization of the Kibara belt and where malachite, bornite, chalcopyrite, sphalerite, chalcocite and covellite were found as ore minerals. Chalcopyrite(±sphalerite) and bornite are clearly replaced by malachite and chalcocite which is also replaced
lately by covellite. These granitoids have been affected by alteration (or contamination) and hence increasing softly the amount of SiO2 compared to unaltered ones as demonstrated by using trace elements plots specially Ta versus Sn, U versus Th and Ta versus Zr/Hf plots. Tin minerals in association with arsenopyrite implies hydrothermal contribution to mineral deposition process.
Despite genetic dissonances, material source of these granitoids could be crustal materials (Al rich-sediments) involved in the collisional process and hence through the partial melting of that crust at utmost conditions of temperature and pressure (high metamorphic degree) produced a felsic magma during the Kibara collisional tectonomagmatic events.
Language
eng
URI
https://hdl.handle.net/10371/151600
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