Late Ediacaran post-collisional magmatism in the Guéra Massif, South-Central Chad

The Guéra Massif of South-Central Chad is composed of collisional and post-collisional intrusive rocks related to the Central African Orogenic Belt. Collisional granites were emplaced at 595–590 Ma but there are a series of post-collisional intrusions that were emplaced at 570–560 Ma. The older granites have zircon U-Pb weighted-mean ages of 569 ± 6 Ma, 568 ± 7 Ma, and 568 ± 6 Ma, whereas the younger granites are 556 ± 7 Ma and 561 ± 6 Ma but all ages are within their maximum uncertainty. Both groups of post-collisional granites are peraluminous, ferroan, and alkali calcic to calc-alkalic but dioritic microgranular enclaves were observed within the older granites. The chemical variability of each group can be explained by fractional crystallization and it is likely that their parental magmas were derived by partial melting of the lower crust. The Nd isotopes of the older rocks (εNd(t) = −9.5 to −1.2) are slightly less enriched than the younger rocks (εNd(t) = −13.1 and −9.1). Moreover, the Nd isotopic compositions appear to indicate the existence of two isotopically distinct regions of the Guéra Massif with one centred on the Mongo (west) region and the other near the El Hidjer to Zan (east) region. The older group was emplaced during a period of crustal thinning after oblique collision and shearing related to the final suturing of the Congo-Saõ Francisco Craton and the Saharan Metacraton. The younger group was likely emplaced after a period of regional WNW-ESE directed crustal shortening. The punctuated nature of deformation/crustal relaxation cycles in the southern Saharan Metacraton suggests that there may have been many small terranes rather than larger cratonic blocks involved in the lithotectonic evolution of the Central African Orogenic Belt north of the Congo-Saõ Francisco Craton.

乍得中南部之古拉山脉由与中非造山带相关的碰撞及碰撞后侵入岩构成。碰撞成因的花岗岩于595至590百万年前形成，而一系列碰撞后侵入岩则于570至560百万年前形成。较老的花岗岩锆石U-Pb加权平均年龄分别为569±6百万年、568±7百万年和568±6百万年，而较年轻的花岗岩年龄则为556±7百万年和561±6百万年，但所有年龄均在其最大不确定性范围内。两组碰撞后花岗岩均为过铝质、含铁质及碱钙质至钙碱性，但在较老的花岗岩中观察到二辉微晶包体。各组的化学变异性可由分馏结晶解释，且其母岩浆可能来源于下地壳的部分熔融。较老岩石的Nd同位素（εNd(t) = −9.5至−1.2）相对于较年轻的岩石（εNd(t) = −13.1和−9.1）略为贫化。此外，Nd同位素组成似乎表明古拉山脉存在两个同位素不同的区域，一个位于Mongo（西部）区域中心，另一个位于El Hidjer至Zan（东部）区域附近。较老的一组花岗岩是在斜向碰撞和剪切相关的最终缝合线形成后的地壳变薄期形成的。较年轻的一组花岗岩可能在区域性地壳NNW-ESE向缩短期之后形成。南撒哈拉元古克拉通变形/地壳松弛周期断续的特性表明，可能存在许多小构造单元，而非大克拉通块体参与到了刚果-圣弗朗西斯科克拉通北部的中非造山带的地层构造演化。