The late Miocene (11–9 Ma) volcanic rocks of Kef Hahouner, ~40 km NE of Constantine (NE Algeria), are commonly classified as lamproites in literature. However, these rocks are characterized by an anhydrous paragenesis with plagioclase and Mg-rich olivine phenocrysts, set in a groundmass made up of feldspars, pyroxenes and opaque minerals. Thus, we classify the Kef Hahouner rocks as ultrapotassic shoshonites and latites, having K2O N 3 wt.%, K2O/Na2O N 2.5, MgO N 3–4 wt.%, SiO2 b 55–57 wt.% and SiO2/K2O b 15. All the investigated samples showprimitive mantle-normalizedmulti-element patterns typical of orogenic (arctype) magmas, i.e. enriched in LILE (e.g. Cs, Rb and Ba) and LREE (e.g. La/Yb=37–59) with respect to the HFSE, peaks at Pb and troughs at Nb and Ta. Initial isotopic ratios are in the range of 87Sr/86Sr = 0.70874–0.70961, 143Nd/144Nd = 0.51222–0.51223, 206Pb/204Pb = 18.54–18.60, 207Pb/204Pb = 15.62–15.70 and 208Pb/204Pb = 38.88–39.16. The Kef Hahouner volcanic rocks show multi-element patterns similar to the other circum-Mediterranean lamproites and extreme Sr, Nd and Pb isotopic compositions. Nevertheless, the abundant plagioclase, the presence of Al-rich augite coupled with high Al2O3 whole rock compositions (9.6–21.4 wt.%), and the absence of phlogopite are all at inconsistentwith the definition of lamproite.Wereviewed the rocks classified as lamproites worldwide, and found that many of these rocks, as for the Kef Hahouner samples, should be actually defined as “normal” potassic to ultrapotassic volcanic rocks. Even the grouping of lamproites into “orogenic” and “anorogenic” types appears questionable.

Exotic lamproites or normal ultrapotassic rocks? The Late Miocene volcanic rocks from Kef Hahouner, NE Algeria, in the frame of the circum-Mediterranean lamproites / Lustrino, M.; Agostini, S.; Chalal, Y.; Fedele, Lorenzo; Stagno, V.; Colombi, F.; Bouguerra, A.. - In: JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH. - ISSN 0377-0273. - 327:(2016), pp. 539-553. [doi:10.1016/j.jvolgeores.2016.09.021]

Exotic lamproites or normal ultrapotassic rocks? The Late Miocene volcanic rocks from Kef Hahouner, NE Algeria, in the frame of the circum-Mediterranean lamproites

FEDELE, LORENZO;
2016

Abstract

The late Miocene (11–9 Ma) volcanic rocks of Kef Hahouner, ~40 km NE of Constantine (NE Algeria), are commonly classified as lamproites in literature. However, these rocks are characterized by an anhydrous paragenesis with plagioclase and Mg-rich olivine phenocrysts, set in a groundmass made up of feldspars, pyroxenes and opaque minerals. Thus, we classify the Kef Hahouner rocks as ultrapotassic shoshonites and latites, having K2O N 3 wt.%, K2O/Na2O N 2.5, MgO N 3–4 wt.%, SiO2 b 55–57 wt.% and SiO2/K2O b 15. All the investigated samples showprimitive mantle-normalizedmulti-element patterns typical of orogenic (arctype) magmas, i.e. enriched in LILE (e.g. Cs, Rb and Ba) and LREE (e.g. La/Yb=37–59) with respect to the HFSE, peaks at Pb and troughs at Nb and Ta. Initial isotopic ratios are in the range of 87Sr/86Sr = 0.70874–0.70961, 143Nd/144Nd = 0.51222–0.51223, 206Pb/204Pb = 18.54–18.60, 207Pb/204Pb = 15.62–15.70 and 208Pb/204Pb = 38.88–39.16. The Kef Hahouner volcanic rocks show multi-element patterns similar to the other circum-Mediterranean lamproites and extreme Sr, Nd and Pb isotopic compositions. Nevertheless, the abundant plagioclase, the presence of Al-rich augite coupled with high Al2O3 whole rock compositions (9.6–21.4 wt.%), and the absence of phlogopite are all at inconsistentwith the definition of lamproite.Wereviewed the rocks classified as lamproites worldwide, and found that many of these rocks, as for the Kef Hahouner samples, should be actually defined as “normal” potassic to ultrapotassic volcanic rocks. Even the grouping of lamproites into “orogenic” and “anorogenic” types appears questionable.
2016
Exotic lamproites or normal ultrapotassic rocks? The Late Miocene volcanic rocks from Kef Hahouner, NE Algeria, in the frame of the circum-Mediterranean lamproites / Lustrino, M.; Agostini, S.; Chalal, Y.; Fedele, Lorenzo; Stagno, V.; Colombi, F.; Bouguerra, A.. - In: JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH. - ISSN 0377-0273. - 327:(2016), pp. 539-553. [doi:10.1016/j.jvolgeores.2016.09.021]
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11588/660855
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