An Assessment of the Magmatic Conditions of Late Neoproterozoic Collisional and Post-collisional Granites From the Guéra Massif, South-Central Chad

Chi Thi Pham,J Gregory Shellnutt,Meng-Wan Yeh,Yoshiyuki Iizuka

doi:10.3389/feart.2020.00318

Chi Thi Pham, J Gregory Shellnutt + Show 2 more

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https://doi.org/10.3389/feart.2020.00318

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Abstract

The Guera Massif, in South-Central Chad hosts granitic rocks that were emplaced during distinct intervals (595-590 Ma; ~570 Ma; ~560 Ma) of the Late Ediacaran Central African Orogenic Belt. To the northwest of the Guera Massif, younger post-collisional granites (554-545 Ma) are found near Lake Fitri. The older ( 590 Ma) rocks have geochemical characteristics of collisional granites whereas the younger ( 570 Ma) rocks are similar to post-collisional granites. Biotite and amphibole were analyzed to constrain the magmatic conditions of the granites. The biotite from the collisional granites tends to have higher Al and Ti and lower Fe# (Fe#average  0.67) than the post-collisional granites (Fe#average  0.88). The average crystallization temperatures range from 696 ± 37oC to 612 ± 8oC, with the average pressure of crystallization from 0.25 ± 0.09 GPa to 0.13 ± 0.02 GPa, and redox conditions between the nickel-nickel oxide (NNO) and quartz-fayalite-magnetite (QFM) buffers. The biotite crystallization temperatures of the post-collisional rocks are generally lower than the collisional rocks (570 Ma = 630 ± 26oC to 619 ± 30oC, 560 Ma = 626 ± 20oC to 607 ± 4oC, 550 Ma = 639 ± 18oC to 612 ± 13oC), but the crystallization pressures are similar (0.27 ± 0.05 GPa to 0.14 ± 0.04 GPa). The redox conditions transition from the QFM buffer to the wustite-magnetite (WM) buffer. In contrast, the biotite from the Lake Fitri post-collisional granites crystallized at higher pressure (0.39 ± 0.08 GPa to 0.35 ± 0.03 GPa) but similar redox conditions. The amphiboles in the younger (~590 Ma) collisional granites and the post-collisional granites yielded crystallization pressure estimates that are generally higher (~0.6 GPa to ~0.1 GPa) than the biotite estimates but there is overlap. The difference in pressure may be due to the timing of crystallization and/or crystal redistribution. Overall, there appears to be a secular change from high to low temperature and pressure whereas the redox conditions appear to be spatially related. The biotite crystallization pressure of the Lake Fitri granites suggests they were likely emplaced into a different domain/terrane of the Saharan Metacraton than the Guera Massif.

Highlights

The Saharan Metacraton occupies the North-Central part of Africa and covers an area of ∼5 million km2 (Abdelsalam et al, 2002; Liégeois et al, 2013)
We present the chemistry of biotite and amphibole from the collision-related and post-collisional granites of the Guéra Massif with the aim to: (1) describe the mineralchemical evolution of the mafic minerals from the granites of the Guéra Massif; (2) estimate magmatic conditions that prevailed during the crystallization of biotite; (3) characterize the geochemical differences in biotite between the collisional and the post-collisional granites, and (4) discuss the tectonomagmatic implications
To investigate the mineralogical composition of the Gueìra Massif granites a total of twelve samples of biotite-granite and hornblende-biotite-granite were selected for analysis by electron probe micro-analyzer (EPMA)

Summary

Introduction

The Saharan Metacraton occupies the North-Central part of Africa and covers an area of ∼5 million km (Abdelsalam et al, 2002; Liégeois et al, 2013). It is surrounded by the Tuareg Shield to the west, the Arabian-Nubian Shield (ANS) to the east, the Congo Craton to the south, and the African continental margin to the north (Figure 1A). The eastern boundary is considered to be the north-trending Keraf– Kabus–Sekerr Shear Zone which is an arc-continental suture that separates the Saharan Metacraton from the ANS (Abdelsalam and Dawoud, 1991; Stern, 1994; Abdelsalam and Stern, 1996). The northern boundary of the Saharan Metacraton is unconstrained but is likely located in southern Egypt and Libya and covered by thick Phanerozoic sediments (Abdelsalam et al, 2002)

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