Chemical mineralogy of the Monchique alkaline complex, southern Portugal

Abstract

Some 500 microprobe analyses from 43 rock samples, covering the entire range of major and minor intrusive rock-types, define the following ranges of mineral composition: plagioclases (An70-30), alkali feldspars (Or98-20 and Ab90–99), olivines (Fo85-82), clinopyroxenes (aluminous titanaugite through salite and aegirine-augite to acmite), amphiboles (subsilicic kaersutite through pargasite to hastingsite, edenite or katophorite), biotites (titanbiotite to titaniferous manganiferous lepidomelane). Varied discontinuous reaction relationships are evident petrographically between these minerals (e.g. amphibole overgrowths on pyroxenes or biotites), but most appear to reflect reequilibration during slow magmatic cooling, or perhaps local disequilibrium effects, and there is little evidence for significant subsolidus, hydrothermal or deuteric modification of the primary mineralogy.

Although these mineral ranges are reasonably typical of differentiated alkaline gabbroic-syenitic intrusions, Monchique also shows many unusual features: e.g. a restricted stability range for olivine, the absence of amphiboles from all rocks with 54<%SiO2<58, a lack of alkali amphiboles corresponding to the acmite-rich pyroxenes, the presence of acmite-poor pyroxenes and aluminous biotites in peralkaline rocks, and irregular Ti variation in biotites. Mineral/ host-rock relationships also show peculiarities: e.g. Mg/ (Mg+Fe) ratios of mafic minerals and An contents of plagioclases increase as host-rock fractionation index (FI) increases from gabbroic (FI 30–40%) to alkali feldspar-bearing essexitic (FI c. 50) rocks. Thereafter, Mg ratios decrease only slightly, such that many malignites and miaskitic syenites (FI 70–90) carry pyroxenes and biotites which are no more, and sometimes less evolved than those in the gabbroic rocks (FI 30–40).

Such features confirm earlier suggestions from whole-rock geochemistry, that a major evolutionary process in the complex was a relatively discontinuous ‘segregation’ of already somewhat evolved essexitic magma into more basic and more felsic magmas, rather than an incremental gabbro→syenite crystal fractionation. They also confirm that the complex represents neither a single in situ differentiating magma pulse, nor a series of simply related pulses, but an irregular and largely irresolvable juxtaposition of magma-batches of widely varying compositions and evolutionary histories.

This is a preview of subscription content, log in to check access.

References

  1. Airos Barros L (1975) Bilan minéralogique et géochimique de l'altération dans un profil de syénite néphelinique de Monchique. Bull Soc Geol Port 19:117–129

    Google Scholar 

  2. Bailey DK (1969) The stability of acmite in the presence of H2O. Am J Sci Schairer Vol 267A:1–16

    Google Scholar 

  3. Canilho MH, Salvado Canelhas MG, Pereira Mano J (1978) Mineralogical investigations of the Monchique nephelines. Bol Soc Geol Portugal 21:83–102

    Google Scholar 

  4. Chatterjee SC (1974) Petrography of the igneous and metamorphic rocks of India. Macmillan Bombay

    Google Scholar 

  5. Czygan W (1969) Petrographie und Alkali-Verteilung im Foyait der Serra de Monchique. Neues Jahrb Mineral Abh 111:32–73

    Google Scholar 

  6. Deer WA, Howie RA, Zussman J (1962) Rock-forming minerals 5 Vols. Longman London, 1st Ed

    Google Scholar 

  7. Deer WA, Howie RA, Zussman J (1978) Rock-forming minerals, Vol 2A. Longman London, 2nd Ed

    Google Scholar 

  8. Frisch T (1970) Chemical variation among the amphiboles of Shefford Mountain: a Monteregian intrusion in southern Quebec. Can Mineral 10:553–570

    Google Scholar 

  9. Frisch T (1973) Clinopyroxenes from Shefford Mountain: a Monteregian intrusion in southern Quebec. Bull Geol Soc Am 84:649–656

    Google Scholar 

  10. Gandhi SS (1970) The petrology of Mt. Yamaska. Can Mineral 10:452–484

    Google Scholar 

  11. IMA (International Mineralogical Association) (1978) Nomenclature of amphiboles. Miner Mag 42:533–563

    Google Scholar 

  12. Kushiro I (1975) On the nature of silicate melt and its significance in magma genesis... Am J Sci 275:411–431

    Google Scholar 

  13. Mitchell RH, Platt GR (1978) Mafic mineralogy of ferroaugite syenite from the Coldwell Alkaline complex, Ontario, Canada. J Petrol 19:627–652

    Google Scholar 

  14. Neumann E-R (1976) Compositional relations among pyroxenes, amphiboles and other mafic phases in the Oslo Region plutonic rocks. Lithos 9:85–109

    Google Scholar 

  15. Nolan J (1966) Melting relations in the system Ab-Ne-Ac-Di-H2O and their bearing on the genesis of alkaline undersaturated rocks. J Geol Soc Lond 122:119–157

    Google Scholar 

  16. Parsons I (1979) The Klokken gabbro-syenite complex, south Greenland: cryptic variation and origin of inversely graded layering. J Petrol 20:653–694

    Google Scholar 

  17. Parsons I (1980) Alkali-feldspars and Fe-Ti oxide exsolution textures as indicators of the distribution and subsolidus effects of magmatic ‘water’ in the Klokken layered intrusion, South Greenland. Trans R Soc Edinburgh Earth Sci 71:1–12

    Google Scholar 

  18. Parsons I (1981) The Klokken gabbro-syenite complex, South Greenland: quantitative interpretation of mineral chemistry. J Petrol 22:233–261

    Google Scholar 

  19. Powell M (1978) The crystallisation history of the Igderfigssalik nepheline syenite intrusion, Greenland. Lithos 11:99–120

    Google Scholar 

  20. Ribbe RH (1975) Feldspar Mineralogy. Min Soc Am Short Course Notes Vol 2.

  21. Rock NMS (1976a) The comparative strontium isotopic composition of alkaline rocks: new data from southern Portugal and East Africa. Contrib Mineral Petrol 56:205–228

    Google Scholar 

  22. Rocks NMS (1976b) Fenitisation around the Monchique alkaline complex, Portugal. Lithos 9:263–279

    Google Scholar 

  23. Rock NMS (1978) Petrology and petrogenesis of the Monchique alkaline complex, southern Portugal. J Petrol 19:171–214

    Google Scholar 

  24. Rock NMS (1979) Petrology and origin of the type monchiquites and associated lamprophyres of Serra de Monchique, Portugal. Trans R Soc Edinburgh 70:149–170

    Google Scholar 

  25. Rock NMS (1982) The Late Cretaceous alkaline igneous province in the Iberian Peninsula, and its tectonic significance. Lithos 15:111–131

    Google Scholar 

  26. Rock NMS (1983) Alguns aspectos geológicos e petrológicos do complexo alcalino de Monchique (in Portuguese). Lisbon, in press

  27. Rock NMS, Leake BE (1982) A FORTRAN computer programme for the classification of amphiboles according to the IMA (1978) scheme. IGS Petrogr Rept 5014 (unpubl)

  28. Santos AR dos (1973) Estudo geológico e geoquímico do maciço de Monchique. Bol Mus Lab Min Geol Univ Lisboa 13:143–251

    Google Scholar 

  29. Scott PW (1976) Crystallisation trends of pyroxenes from the alkaline volcanic rocks of Tenerife, Canary Islands. Miner Mag 40:805–816

    Google Scholar 

  30. Sorensen H (1974) The alkaline rocks. New York, Wiley

    Google Scholar 

  31. Sousa F Pereira de (1926) La Serra de Monchique. Bull Soc Geol Fr 4 Ser, 26:321–350

    Google Scholar 

  32. Stephenson D (1972) Alkali clinopyroxenes from nepheline syenites of the South Qoroq centre, south Greenland. Lithos 5:187–201

    Google Scholar 

  33. Stephenson D (1974) Mn & Ca-enriched olivines from nepheline syenites of the South Qoroq centre, south Greenland. Lithos 7:35–1

    Google Scholar 

  34. Stephenson D, Upton BGJ (1982) Ferromagnesian silicates from a differentiated alkaline complex; Kungnat Fjeld, South Greenland. Mineral Mag 46:283–300

    Google Scholar 

  35. Tilley CE (1954) Nepheline-alkali feldspar parageneses. Am J Sci 252:65–75

    Google Scholar 

  36. Valiquette G, Archambault G (1970) Les gabbros et les syénites du complexe de Brome. Can Mineral 10:485–510

    Google Scholar 

  37. Woussen G (1970) La géologie du complexe igné du Mont Royal. Can Mineral 10:432–451

    Google Scholar 

  38. Yagi K (1953) Petrochemical studies on the alkalic rocks of the Morutu district, Sakhalin. Bull Geol Soc Am 64:769–810

    Google Scholar 

  39. Yagi K (1966) The system acmite-diopside, and its bearing on the stability relations of natural pyroxenes of the acmite-hedenbergite series. Am Mineral 51:976–1000

    Google Scholar 

Download references

Author information

Affiliations

Authors

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Rock, N.M.S. Chemical mineralogy of the Monchique alkaline complex, southern Portugal. Contr. Mineral. and Petrol. 81, 64–78 (1982). https://doi.org/10.1007/BF00371160

Download citation

Keywords

  • Olivine
  • Fractionation Index
  • Alkali Feldspar
  • Mafic Mineral
  • Gabbroic Rock