Ca-rich pyroxene from basic and silicic volcanic rocks from the Cameroon Volcanic Line (West-Africa): crystal chemistry and petrological relationships

C2/c pyroxene (Cpx) phenocrysts from basic rocks (basanite-trachyphonolite suite and alkali basalts) and differentiated CIPW Ne- and Q-normative trach ytes and rhyolites occurring in the continental sector of the Cameroon Volc anic Line were investigated by single-crystal X-ray diffraction combined wi th electron probe microanalysis. This technique provides accurate data on s ite occupancy and geometric variations. The investigated Cpx is the only liquidus Ca-rich crystallizing phase from both basic and differentiated volcanics. Cpx from basic alkaline rocks are characterized by large V-T owing to their low Si(T), which requires high R3+ (Al-VI + Fe3+ + Cr3+ + Ti4+) in M1 site , due to Si-Al-IV substitution. High Ca content allows the appropriate netc harge for the overbonded O3 oxygens. Cpx from differentiated silicic volcanics have large V-M1 related to high F e2+ (M1), since Cpx crystallized from MgO-poor (< 0.5 wt%) melts at relativ ely low fO(2) (QFM buffer). The high Fe2+ (M1) causes lengthening of [M1-O] , which requires shortening of T-O1 and T-O2 (i.e. high Si), irrespective o f the magmatic affinity of the Cpx in SiO2-undersaturated or oversaturated conditions. Therefore, Cpx from silicic rocks have similar (Ca + Na) conten ts to those formed from basic rocks, but their [M2-O3] is relatively longer , owing to the shortening of [T-O3]. It follows that Cpx crystallizing from silicic melts show a relatively large V-M2 increase, which is not related to an increase in (Ca + Na). The beta-Delta(M2) (distortion parameter) relationship of Cpx indicates tha t the basic magmas parental to Q-normative rocks were slightly to moderatel y alkaline basalts and not SiO2-undersaturated or oversaturated, while thos e from trachyphonolite and Netrachyte were distinctly more SiO2-undersatura ted (e.g. basanitic magmas). V-cell-V-M1 relationships confirmed that the investigated Cpx phenocrysts c rystallized in the crust at relatively low pressure (ca. 1-5 kbar).