THE CENTRAL HOGGAR-TAOURIRT AND ALBITE-TOPAZ POST PAN-AFRICAN GRANITES (SOUTHERN ALGERIA) - THEIR PETROLOGY, GEOCHEMISTRY AND PETROGENESIS

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) granite s are also peraluminous but are siderophyllitic biotite- and hornblend e-bearing rocks formed from a volatile poorer melt compared with the A T granites but the aplopegmatitic stockscheiders are genetically relat ed 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 (Sigma(VI) = 4.76 +/- 0. 27 a.p.f.u.). The substitutions 2Fe(2+) (2Mg(2+))(Ti4+) <--> Al-IV + L i commonly found in biotites occurring in similar granitoids were insi gnificant during the crystallization of biotite in the Hoggar AT grani tes 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 2Fe(2+)(2Mg(2+))(Ti4+) <-> Al-VI Li, and 2Al <-> 3Fe(2+) (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 pattern s (Ce/Yb)(CN) = 0.76 +/- 0.52, and have formed by fractionation of mai nly plagiodase +/- biotite +/- K-feIdspar +/- monazite. Modelling of R b, Ba, Sr, La, Ce, Sm, Eu, Yb, Lu, Y and (Sr-87/Sr-86) isotopic ratios indicates that the two granites do not represent simple segregated me lts, they were not generated by combined wall rock assimilation-fracti onal 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 coul d be explained by subduction related magmatism whereas the the post Pa n-African granites melts produced in the lower crust from heterogeneou s sources.