Isotopic (O, Sr, Nd) and trace element geochemistry of the Laouni layered intrusions (Pan-African belt, Hoggar, Algeria): evidence for post-collisional continental tholeiitic magmas variably contaminated by continental crust

The three layered intrusions studied in the Laouni area have been emplaced within syn-kinematic Pan-African granites and older metamorphic rocks. They have crystallized at the end of the regional high-temperature metamorphism , but are free from metamorphic recrystallization, revealing a post-collisi onal character. The cumulate piles can be interpreted in terms of two magma tic liquid lines of descent: one is tholeiitic and marked by plagioclase-ol ivine-clinopyroxene cumulates (troctolites or olivine bearing gabbros), whi le the other is calc-alkaline and produced orthopyroxene-plagioclase rich c umulates (norites), One intrusion (WL (West Laouni)-troctolitic massif), sh ows a Lower Banded Zone where olivine-chromite orthocumulates are interlaye red with orthopyroxene-rich and olivine-plagioclase-clinopyroxene cumulates , whereas the Upper Massive Zone consists mainly of troctolitic and gabbroi c cumulates. The other two massifs are more homogeneous: the WL-noritic mas sif has a calc-allkaline differentiation trend whereas the EL (East Laouni) -troctolitic massif has a tholeiitic one. Separated pyroxene and plagioclas e display similar incompatible trace element patterns, regardless of the cu mulate type. Calculated liquids in equilibrium with the two pyroxenes for b oth noritic and troctolitic cumulates an characterized by negative Nb, Ta, Zr and Hf anomalies and light REE enrichment inherited from the parental ma gmas. Troctolitic cumulates have mantle-derived delta(18)O (+ 5 to +6 parts per thousand), initial Sr-87/Sr-86 (Sri = 0.7030 to 0.7054), epsilon(Nd) ( +5 to -1) values whereas noritic cumulates are variably enriched in delta(1 8)O (+7 to +9 parts per thousand), show negative epsilon(Nd) (-7 to -12) an d slightly higher Sr, (0.7040-0.7065). Based on field, isotopic ratios are interpreted as resulting from a depleted mantle source (Sr-i = 0.7030; epsi lon(Nd) = +5.1; delta(18)O = +5.1 parts per thousand) having experience sho rt term incompatible element enrichment and Variable crustal contamination. The mantle magma was slightly contaminated by an Archaean lower crust in t roctolitic cumulates, more strongly and with an additional contamination by an Eburnian upper crust in noritic cumulates. Lower crust input is recorde d mainly by Sr and Nd isotopes and upper crust input by O isotopes. This is probably due to the different water/rock ratios of these two crust types. Assimilation of low amounts (< 10%) of quartz-bearing felsic rocks, coming from both lower and upper crust, can explain the rise of SiO2 activity, the enrichment in O-18 and Sr-87 and the lowering of epsilon(Nd) in the noriti c cumulates compared to troctolitic ones. The geodynamic model proposed to account for the Laouni tholeiitic magmatism involves a late Pan-African ast henospheric rise due to a rapid lithospheric thinning associated with funct ioning of shear zones, which allowed tholeiitic magmas to reach high crusta l levels while experiencing decreasing degrees of crustal contamination wit h time. (C) 1998 Elsevier Science B.V. All rights reserved.