Basic ordovician magmatism in Northeastern Puna: similarities and differences among the Cobres and Tanque Ranges, Salta and Jujuy.

COIRA, B.; KIRSCHBAUM, A.; PÉREZ, B.; HONGN, F. AND N., MENEGATTI

Mendoza, Argentina.

Simposio; 12th International Gondwana Symposium. “Geological and Biological Heritage of Gondwana”.; 2005

Academia Nacional de Ciencias

Basic magmatism related to the Cobres and Tanque Ordovician volcano-plutonic complexes in the northeastern Puna, at 23º–24ºS, is analyzed and compared in order to identify and characterize the different magmatic pulses that occurred during the geological evolution of the region. Records of basic magmatism are recognized in Early Ordovician sedimentary-volcanic sequences which host the plutonic complexes of Cobres and Tanque, as well as minor members of those complexes and as post-granite dykes.                                                                                                                        In the Cobres range (western Salinas Grandes) the pre- and syn-granite Ordovician gabbros are composed essentially of hornblende and plagioclase with apatite, opaques, and zircon, and exhibit subalkaline geochemical characteristics with low TiO2 (0.2–0.9%) and P2O5. Diagrams of trace elements normalized to MORB show depletion of Nb in relation to La, Ce, and Th, and some enrichment in LILE, particularly Th, Cs and Ba; these are characteristics of arc basalts. At the same time they show a REE pattern very similar to MORB or slightly enriched in LREE. Meanwhile the post-granite gabbros and basalts are alkaline and constitute two groups. One group is characterized by plagioclase, hornblende, clinopyroxene, titano-ilmenite mineralogy and moderate alkalinity with high TiO2 (2.4–2.6%), P2O5, Nb, Ta, Hf, Ba and Sr contents, high La/Yb ratios (11–12) and low La/Ta (11–13). The other group is represented by trachybasalts, tephrites and lamprophyres with alkaline mafic minerals and feldspathoids with a stronger alkaline signature, higher incompatible element contents as well as higher P2O5, Nb and Ta concentrations than the previous group, and La/Yb (28) ratios with LREE enrichment and similar La/Ta ratios, comparable to Early Cretaceous basic members of the Sierra Rangel Alkaline Complex.                                                                                                                                                    In the Tanque range (western Cobres range), basic and siliceous lava flows, sills and laccoliths are represented in an Early Ordovician sedimentary volcanic sequence which hosts the syntectonic Tanque Granite. Meanwhile gabbros of the plutonic complex show mingling and mixing relationships with the granite. Lastly, stocks and dykes are recognized intruding the granite with sharp contacts. The pre-granite basalts and microgabbros are composed of plagioclase and clinopyroxene or hornblende, in both cases partly replaced by tremolite-actinolite and with titanite, apatite and opaques; they have subalkaline or alkaline signatures. The former are geochemically comparable to the pre- and syn-granite gabbros of Cobres, whereas the latter are alkaline, with an intraplate signature (La/Ta=13), and enriched in LREE, with La/Yb ratios = 8 to 10. The syn-granite gabbros are composed of plagioclase, hornblende and opaques and show a more alkaline signature than the last group, with SiO2= 46-51%, higher TiO2 (2.4–2.8%), P2O5 (0.4–0.6%), LIL, Nb, Ta, Hf, LREE, La/Yb=18–20 and La/Ta= 10–11. Post-granite alkaline lamprophyres to alkali gabbros stocks can be differentiated from the syn-granite alkali gabbros on mineralogical and geochemical grounds. They contain titanaugite, kaersutite, plagioclase and opaques, and are characterized by lower SiO2 (40–45.5%), higher TiO2 (4.7–5.2%) and P2O5 (1.5–2.2%), lower LIL, Nb, Zr, LREE, and higher Nd contents. Postgranite dykes are of two types: one similar to the pre-granite alkaline basalts and the other represented by tephrites, trachybasalts and alkali lamprophyres with clinopyroxene, barkevikite, kaersutite, minor serpentinized olivine, and feldspathoids. These last are rich in TiO2 (2.3–3%) and K2O (1.5–3.1%) and have the highest LIL, Nb, Ta, Hf contents and steepest REE patterns (La/Yb=29–52) observed in Tanque. Their characteristics enable them to be correlated with basic members of the Sierra Rangel Alkaline Complex.                                                                                                                                                                            The geochemical variations observed among the basic rocks that have been erupted during the three main pulses recognized in the Ordovician evolution of the Cobres and Tanque volcano-plutonic complexes might be correlated with changes in the pressure for comparable degrees of melting of similar enriched sources, reflecting deeper conditions of magma segregation in the case of the Tanque complex, where alkali members are recorded in the Early Ordovician volcano-sedimentary host rock as well in the syn- and post-granite basic members. In contrast, in the Cobres complex basalts and gabbros pre- and syn-granite show subalkaline affinities and smaller volumes. Other possibility to consider is a variation in the degrees of melting, with higher percentages in the case of subalkaline gabbros and basalts of Cobres.2 (0.2–0.9%) and P2O5. Diagrams of trace elements normalized to MORB show depletion of Nb in relation to La, Ce, and Th, and some enrichment in LILE, particularly Th, Cs and Ba; these are characteristics of arc basalts. At the same time they show a REE pattern very similar to MORB or slightly enriched in LREE. Meanwhile the post-granite gabbros and basalts are alkaline and constitute two groups. One group is characterized by plagioclase, hornblende, clinopyroxene, titano-ilmenite mineralogy and moderate alkalinity with high TiO2 (2.4–2.6%), P2O5, Nb, Ta, Hf, Ba and Sr contents, high La/Yb ratios (11–12) and low La/Ta (11–13). The other group is represented by trachybasalts, tephrites and lamprophyres with alkaline mafic minerals and feldspathoids with a stronger alkaline signature, higher incompatible element contents as well as higher P2O5, Nb and Ta concentrations than the previous group, and La/Yb (28) ratios with LREE enrichment and similar La/Ta ratios, comparable to Early Cretaceous basic members of the Sierra Rangel Alkaline Complex.                                                                                                                                                    In the Tanque range (western Cobres range), basic and siliceous lava flows, sills and laccoliths are represented in an Early Ordovician sedimentary volcanic sequence which hosts the syntectonic Tanque Granite. Meanwhile gabbros of the plutonic complex show mingling and mixing relationships with the granite. Lastly, stocks and dykes are recognized intruding the granite with sharp contacts. The pre-granite basalts and microgabbros are composed of plagioclase and clinopyroxene or hornblende, in both cases partly replaced by tremolite-actinolite and with titanite, apatite and opaques; they have subalkaline or alkaline signatures. The former are geochemically comparable to the pre- and syn-granite gabbros of Cobres, whereas the latter are alkaline, with an intraplate signature (La/Ta=13), and enriched in LREE, with La/Yb ratios = 8 to 10. The syn-granite gabbros are composed of plagioclase, hornblende and opaques and show a more alkaline signature than the last group, with SiO2= 46-51%, higher TiO2 (2.4–2.8%), P2O5 (0.4–0.6%), LIL, Nb, Ta, Hf, LREE, La/Yb=18–20 and La/Ta= 10–11. Post-granite alkaline lamprophyres to alkali gabbros stocks can be differentiated from the syn-granite alkali gabbros on mineralogical and geochemical grounds. They contain titanaugite, kaersutite, plagioclase and opaques, and are characterized by lower SiO2 (40–45.5%), higher TiO2 (4.7–5.2%) and P2O5 (1.5–2.2%), lower LIL, Nb, Zr, LREE, and higher Nd contents. Postgranite dykes are of two types: one similar to the pre-granite alkaline basalts and the other represented by tephrites, trachybasalts and alkali lamprophyres with clinopyroxene, barkevikite, kaersutite, minor serpentinized olivine, and feldspathoids. These last are rich in TiO2 (2.3–3%) and K2O (1.5–3.1%) and have the highest LIL, Nb, Ta, Hf contents and steepest REE patterns (La/Yb=29–52) observed in Tanque. Their characteristics enable them to be correlated with basic members of the Sierra Rangel Alkaline Complex.                                                                                                                                                                            The geochemical variations observed among the basic rocks that have been erupted during the three main pulses recognized in the Ordovician evolution of the Cobres and Tanque volcano-plutonic complexes might be correlated with changes in the pressure for comparable degrees of melting of similar enriched sources, reflecting deeper conditions of magma segregation in the case of the Tanque complex, where alkali members are recorded in the Early Ordovician volcano-sedimentary host rock as well in the syn- and post-granite basic members. In contrast, in the Cobres complex basalts and gabbros pre- and syn-granite show subalkaline affinities and smaller volumes. Other possibility to consider is a variation in the degrees of melting, with higher percentages in the case of subalkaline gabbros and basalts of Cobres.