Timing and petrogenesis of collisional and post-collisional rocks of Guéra Massif, Republic of Chad

Timing and petrogenesis of collisional and post-collisional rocks of Guéra Massif, Republic of Chad

范氏玉河(Pham Thi Ngoc Ha)

指導教授 ： Shellnutt ； John Gregory

國立臺灣師範大學/理學院/地球科學系/碩士(2018年)

https://doi.org/10.6345/THE.NTNU.DES.002.2018.B07

摘要

None

關鍵字

Granites ； Guéra Massif ； Pan-African Orogeny ； Saharan Metacraton

並列摘要

The poorly-studied Saharan Metacraton of North-Central Africa is located between the Arabian-Nubian Shield in the east, the Tuareg Shield in the west and the Central African Orogenic Belt in the south. The Republic of Chad, constrained within the Saharan Metacraton, comprises a Phanerozoic cover overlying Precambrian basement outcroppings in five distinct massifs: The Mayo Kebbi, Tibesti, Ouaddaï, Yadé and the Guéra. The Guéra Massif is the least studied of the five massifs but it likely preserves structures that were formed during the collision between Congo Craton and Saharan Metacraton. It is composed of diverse igneous rock types including granites, rhyolites, and gabbros. The first zircon U/Pb geochronology of the silicic rocks from the Guéra Massif yielded three main age groups: ~590 Ma, ~570 Ma, ~560 Ma, while a single gabbro yielded an age of ~580 Ma.
The ~590 Ma granites are metaluminous to peraluminous, alkali calcic to calc-alkalic and magnesian in composition. They are similar to granitic rocks typical of cordilleran or volcanic-arc settings (collisional rocks). The younger (≤ 570 Ma) granites and rhyolite are peraluminous to metaluminous, alkali calcic to calc-alkalic but ferroan and are similar in compositions to post-collisional granites (high K, calc-alkaline). Both magnesian and ferroan rocks show the enrichment in LREE with distinct negative Eu anomalies (Eu/Eu* = 0.04-0.88). The negative εNd(t) isotopes (εNd(t) = -14.5 to -0.2), variable ISr values (0.7037-0.7176), low TZr (~700oC) and abundant inherited zircons indicate the granitic rocks are derived primarily from crustal sources.
The ~580 Ma gabbro has a similar age and composition as a mafic, within-plate alkaline complex in West-Cameroon and matches the age of inherited zircons from different rock types around Lake Chad and Lake Fitri suggesting that there was a regional magmatic event at that time despite the dearth of rocks. The major and trace element geochemistry of the gabbro is similar to alkaline within-plate basalt suggesting that it was derived from a garnet-bearing mantle source. The enriched Sr-Nd isotopes (ISr = 0.7056, εNd(t) = -4.6) with the lack of crustal contamination indicates that it may originate from an enriched mantle (EMII) reservoir.
The geochronology and geochemical data indicate that there is a temporal evolution in the composition of rocks with the old, high εNd(t), magnesian collisional granites shifting to young, low εNd(t), ferroan post-collisional granites. The results suggest that the granites in the Guéra Massif were probably derived by the repeated melting of a single crustal source region, from an initially ‘fertile’ source to a ‘less fertile’ source at distinct intervals. The enriched isotopic composition of the Guéra Massif granites contrasts with the depleted compositions of the associated Neoproterozoic granites in southern Chad (Mayo Kebbi Massif, Lake Fitri inliers) indicates a thicker crustal root may exist under the Guéra Massif. The emplacement of different rock types in the Guéra Massif marks the tectonic regime transition from subduction and collision to post-collision within a span of ~40 million years.

並列關鍵字

Granites ； Guéra Massif ； Pan-African Orogeny ； Saharan Metacraton

參考文獻

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