Alkaline rocks.lamprophyre,lamproite,and carbonatites

Igneous petrology

1. LAMPROPHYRE,LAMPROITE AND CARBONATITE (A GROUP OF ALKALINE ROCKS) NIHARIKA NAIDU  M.Sc. SECOND SEMESTER

2. CONTENTS Introduction Mineralogy Geochemistry Classification Petrogenesis reference

3. LAMPROPHYRE

4. INTRODUCTION Lamprophyre derived from Greek word LAMPROS meaning bright and phyro meaning to mix. Lamprophyre are melanocratic, porphyritic ,hypabyssal, ultramafic to mafic igneous rock.  Lamprophyre are uncommon small-volume  ultrapotassic igneous rock primarily occurring as dikes lopoliths laccoliths stock and small intrusion. It has a panidiomorphic granular texture.

5. MINERALOGY The lamprophyre commonly consists of alkali rich calc alkali to ultramafic mineral. The essential mineral composition of lamprophyre are biotite amphibole pyroxene feldspar or feldspathoid. The accessory mineral compositions of lamprophyre are sulphide,zeolite,fe-ti oxides and apatite or olivine.

6. GEOCHEMISTRY Lamprophyres are intrusive dyke rock ,they do not have common plutonic or volcanic equivalents. Lamprophyres have variable chemical composition but in genral they have high alkali. In some varieties particularly high k2o and comparetively low silica. The carry abundance volatile bearing minerals and accordingly their analyses show high content of h2o,co2,p2o5,s etc.

7. The lamprophyre that beling to these groups,and the mineral characteristics of each ,as recommended by the IUGS, are givn in the the following table,

8. CLASSIFICATION Lamprophyre are classified into three types -   calc-alkaline lamprophyre    Alkaline lamprophyre    Melilitic lamprophyre

9. CALC ALKALINE LAMPROPHYRE The calc-alkaline lamprophyres are the common lamprophyres and often described as ordinary lamprophyres. This four lamprophyres are most readily classified using modal data:  VOGESITES- From Vosges in Northern France. A vogesite is a porphyritic alkaline igneous rock dominated by essential amphibole, usually hornblende, and potassic feldspar, often with augite and plagioclase present as accessories within the groundmass.  MINETTES - From an old term used by miners in the Vosges, are alkali-feldspar. A minette is a porphyritic alkaline igneous rock dominated by essential biotite and potassic feldspar, often with augite and plagioclase present as accessories within the groundmass. 

10. MINETTES - From an old term used by miners in the Vosges, are alkali-feldspar. A minette is a porphyritic alkaline igneous rock dominated by essential biotite and potassic feldspar, often with augite and plagioclase present as accessories within the groundmass.  SPESSARTITES - From Spessart mountains in Germany. A spessartite is a porphyritic alkaline igneous rock dominated by essential amphibole, usually hornblende, and plagioclase feldspar, often with augite present as an accessory. Plagioclase occurs in the groundmass and potassic feldspar is absent or present in low abundance KERSANTITES - From Kersanton , a village in France, are Plagioclase, Honrblende , Augite lamprophyres. 

11. ALKALINE AND MELILITIC LAMPROPHYRES The alkaline and melilitic lamprophyres will be considered together.  Because both groups contain alkaline rocks and are usually associated with alkaline complexes and the rocks of the carbonatite-nepheline ijolite association. COMPTONITE - From Campton in the New Hampshire (USA). A camptonite is a porphyritic alkaline igneous rock dominated by essential plagioclase and brown amphibole, usually hornblende, often with titanaugite . Plagioclase occurs in the groundmass. SANNAITES : From Sannavand , Fen complex, Sweden. Sannaites are broadly to Comptonites , except that they contain alkali feldspar in place of plagioclase

12. Monchiquites : From Sierra de Monchique in Southern Portugal. A monchiquite is a porphyritic alkaline igneous rock dominated by essential olivine, titanaugite and brown hornblende.   Alnoite - From Alno island, Sweden. A alnoite is a porphyritic alkaline igneous rock dominated by essential olivine, biotite and pyroxene, in a groundmass containing melilite. It can contain monticellite.   Polenzite : From Polzen area of the Bohemian massif, Czechoslovakia. Is a melilitic lamprophyre that usually contain between 10-30% of feldspathoids (Nepheline and Hauyne ) and it normally contains the same minerals as occur in alnöite . 

13. PETROGENESIS       The essential components in lamprophyre genesis are; high depth of melting : which yields more mafic magmas. low degrees of partial melting : which yields magmas rich in the alkalis (particularly potassium and sodium). lithophile element (K, Ba, Cs, Rb) enrichment, high Ni and Cr. Volatile enrichment: to provide the biotite (phlogopite) and amphibole (pargasite) mineralogy Lack of fractional crystallisation  (generally; there are exceptions) High Mg#

14. ECONOMIC IMPORTANCE The economic importance of ultrapotassic rocks is wide and varied. Kimberlites, lamproites perhaps even lamprophyres and are known to contain diamond . These rocks are all produced at depths in excess of 120 km and thus can bring diamond to the surface as xenocrysts  Ultrapotassic granites are a known host for much granite-hosted gold porphyry mineralisation . Significant -style mineralisation is won from highly potassic to ultrapotassic granites. Ultrapotassic A-type intracontinental granites may be associated with fluorite and columbite– tantalite mineralization

15. INDIAN OCCURRENCE

16. INDIAN OCCURRENCE In INDIA lamprophyres are mostly from the gondwana basins. Although they have been reported from some younger alkaline complexes also.   From raniganj coal field minette and kersantite are the most frequently reported types   Calc-alkali lamprophyres are reported from mundwana complex of RAJASTHAN   Amba- dongar carbonatite complex of GUJARAT is intruded by camptonite and kersantite

17. LAMPROITE

18. INTRODUCTION The term lamproite is used form a group of lamprophyre like " hypabyssal"  and  extrusive  igneous rock .  Lamproites are  ultrabasic, ultrapotassic  and  perpotassi c  mantle-derived volcanic or subvolcanic rocks.  Lamproite usually contain glass. Have  low  CaO , Al 2 O 3 , Na 2 O, high K 2 O/Al 2 O 3 ,  a relatively high MgO content and extreme enrichment in  incompatible trace elements.  Lamproite volcanology is varied Both  diatreme  styles and  cinder cone or cone  edifices known for lamproites.

19. MINRALOGY The rocks are high in potassium with 6 to 8% K2O. Forsteritic -olivine, high-Fe leucite, Ti-rich and Al-poor  phlogopite,K -rich and Ti-rich richterite, low-Al diopside, spinel and Fe-rich sanidine.  The rocks commonly altered to talc with carbonate  or serpentine, chlorite and magnetite.

20. CLASSIFICATION

21. GEOCHEMISTRY Molar K 2 O/Na 2 O > 3, i.e., ultrapotassic Molar K 2 O/Al 2 O 3 > 6.8 and commonly > 1 Molar K 2 O + Na 2 O/ Al 2 O 3  typically > 1, i.e., peralkaline Typically <10  wt % each of  FeO  and  CaO , TiO 2  1-7  wt %, >2000 and commonly >5000 ppm Ba, >500 ppm Zr, >1000 ppm Sr and >200 ppm La.

22. PETROGENISIS Lamproites form from partially melted mantle at depths exceeding 150 km. Derived from lherzolite, harzburgite or eclogite mantle source.  Genesis involves mantle-metasomatism.  The molten material is forced to the surface in volcanic pipes, sometimes bringing with it xenoliths and diamonds from mantle regions where diamond formation is stabilized. Based on isotope study (Pb-Pb) the source of lamproites may be transition zone melts of subducted lithosphere which has become trapped at the base of the lithospheric mantle. 

23. CARBONATITE

24. INTRODUCTION Extremely enriched in rare earth elements.  An ultrabasic igneous rock that contain carbonate minerals more than 50%.  Carbonatites can be extrusive or intrusive.  Carbonatite primarily composed of calcite ( sovite ) or dolomite ( rauhaugite ). Generally found associated with kimberlite and alkaline rocks.  

25. TEXTURE Corse to fine grain texture in hand specimen. Hypidiomorphic  texture in thin sections.  Fenite  and  Fenitization  is occur associated with carbonatites. Fenite  is the contact zone between carbonate magma and country  rock enriched in  alkalies  i.e. Na and K. Fenite  zone serves  a  indicator zone that marks the presence of a carbonatite. 

26. MINERALOGY

27. CLASSIFICATION

28. REE bearing minerals: Monazite ( Ce,La )[PO 4 ] Bastnaesite Ce[CO 3 ]F Parisite  Ce 2 Ca[CO 3 ] 3 F 2 Rinkolite  CeNa 2 Ca 4 Ti[Si 4 O 15 ]F 3 Nb and Ta bearing minerals: Niobite  ( Fe,Mn )Nb 2 O 6 Tantalite ( Fe,Mn )Ta 2 O 6 Pyrochlore ( Ca,Na ) 2 ( Nb,Ti ) 2 O 6 F Microlilte   CaNa ( Ta,Ti ) 2 O 6 (OH,F) Fergusonite   Y( Nb,Ta )O 4 Loparite     ( Ca,Na,Ce )( Ti,Nb )O 3 Euxenite     ( Y,Ca,Ce,U,Th )( Nb,Ta,Ti ) 2 O 6 Samarskite     ( Y,Ce,U,Ca,Pb )( Nb,Ta,Ti,Sn ) 2 O 6 Simpsonite     Al4( Ta,Nb ) 3 O 13 (OH,F)

29. GEOCHEMISTRY

30. PETROGENISIS Three theories have been given for the origin of carbonatites: Low degree partial melting  Fractional crystallization  Liquid immiscibility

31. ASSOCIATION OF CARBONATITE Associated with alkaline ring complexes: Core carbonatites Carbonatite dykes, ring-dykes, cone-sheets and brecciate carbonatites.  Associated with alkaline complexes (non-ring types): Large tabular to irregular masses. Dykes, dyke swarms, stockwork or sill within alkaline complexes.  Indirect association with alkaline complex: Large sheets of composite structure. Dyke or sill swarms of carbonatite Extrusive or volcanic carbonatites: Carbonatite lava flows. Pyroclastic carbonatites. 

32. WORLD OCCURRENCE OF CARBONATITE