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-collisional character. The cumulate piles can be interpreted in terms of two magmatic liquid lines of descent: one is tholeiitic and marked by plagioclase-olivine-clinopyroxene cumulates (troctolites or olivine bearing gabbros), while the other is calc-alkaline and produced orthopyroxene-plagioclase rich cumulates (norites). One intrusion (WL (West Laouni)-troctolitic massif), shows a Lower Banded Zone where olivine-chromite orthocumulates are interlayered with orthopyroxene-rich and olivine-plagioclase-clinopyroxene cumulates, whereas the Upper Massive Zone consists mainly of troctolitic and gabbroic cumulates. The other two massifs are more homogeneous: the WL-noritic massif has a calc-alkaline differentiation trend whereas the EL (East Laouni)-troctilitic massif has a tholeiitic one. Separated pyroxene and plagioclase display similar incompatible trace element patterns, regardless of the cumulate type. Calculated liquids in equilibrium with the two pyroxenes for both noritic and troctolitic cumulates are characterized by negative Nb, Ta, Zr and Hf anomalies and light REE enrichment inherited from the parental magmas. Troctolitic cumulates have mantle-derived δ18O (+5 to +6‰), initial 87Sr/86Sr (Sr(i) = 0.7030 to 0.7054), ε(Nd) (+5 to -1) values whereas noritic cumulates are variably enriched in δ18O (+7 to +9‰), show negative ε(Nd) (-7 to -12) and slightly higher Sr(i) (0.7040-0.7065). Based on field, isotopic ratios are interpreted as resulting from a depleted mantle source (Sr(i) = 0.7030; ε(Nd) = +5.1; δ18O = +5.1‰) having experience short term incompatible element enrichment and variable crustal contamination. The mantle magma was slightly contaminated by an Archaean lower crust in troctolitic cumulates, more strongly and with an additional contamination by an Eburnian upper crust in noritic cumulates. Lower crust input is recorded 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 18O and 87Sr and the lowering of ε(Nd) in the noritic cumulates compared to troctolitic ones. The geodynamic model proposed to account for the Laouni tholeiitic magmatism involves a late Pan-African asthenospheric rise due to a rapid lithospheric thinning associated with functioning of shear zones, which allowed tholeiitic magmas to reach high crustal levels while experiencing decreasing degrees of crustal contamination with time.
- Continental tholeiites
- Crustal contamination
- Late Pan-African
- Layered ultramafic-mafic intrusions
- Lithospheric thinning