The Central Hoggar Taourirt and albite-topaz post Pan-African granites (southern Algeria); their petrology, geochemistry and petrogenesis

Abstract

Detailed field, petrological and mineralogical studies of two 540-520 Ma post Pan-African granitoids from Central Hoggar, southern Algeria, show them to have two contrasting mineralogies. The topaz-albite (AT) granitoids are peraluminous rocks containing protolithionite and were probably derived from a volatile-rich magma. The Taourirt (T) granites are also peraluminous but are siderophyllitic biotite- and hornblende-bearing rocks formed from a volatile poorer melt compared with the AT granites but the aplopegmatitic stockscheiders are genetically related to these AT granites. The protolithionites are Li- and F-rich (Li2O = 1.86 ± 0.58 wt %; F 3.65 ± 1.19 wt %) Mg-poor (0.12 ± 0.12 wt %) and have many unoccupied octahedral sites (ΣVI = 4.76 ± 0.27 a.p.f.u.). The substitutions 2Fe2+ (2M2+) (Ti4+) ↔ AlIV+Li commonly found in biotites occurring in similar granitoids were insignificant during the crystallization of biotite in the Hoggar AT granites and stockscheiders. The siderophyllitic biotites are Fe-rich (FeO* = 26.57 ± 3.38), relatively rich in Li (Li2O = 0.36 ± 0.15 wt %) and F (1.11 ± 0.68 wt %), and have variable Mg contents (MgO = 1.89 ± 1.01 wt %). The substitutions 2Fe2+ (2Mg2+(Ti4+) ↔ AlVILi, and 2Al ↔ 3Fe2+ (3Mg2+) were important during the crystallization of biotite in these T granites. The T granites, which have steep REE patterns, (Ce/Yb)CN = 5.96 ± 3.14, were generated by fractionation of plagioclase + K-feldspar + biotite + magnetite (ilmenite) ± hornblende ± allanite but the AT granites exhibit flat REE patterns (Ce/Yb)CN = 0.76 ± 0.52, and have formed by fractionation of mainly plagioclase ± biotite ± K-feldspar ± monazite. Modelling of Rb, Ba, Sr, La, Ce, Sm, Eu, Yb, Lu, Y and (87Sr/86Sr) isotopic ratios indicates that the two granites do not represent simple segregated melts, they were not generated by combined wall rock assimilation-fractional crystallization processes of an evolving mantle-derived magma nor do they represent the evolved end-members of the magma from which the spatially associated 670-570 Ma early-late Pan-African granites were crystallized. The chemistry of these early-late orogenic granites could be explained by subduction related magmatism whereas the the post Pan-African granites melts produced in the lower crust from heterogeneous sources.