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Nyamuragira (3058 a.s.l.) is a large volcanic shield characterized by alkaline rocks ranging from basanite to tephrite and rare transiZonal basalt. Nyiragongo is a ...
MINERALOGY, GEOCHEMISTRY AND PETROGENESIS OF NYIRAGONGO AND NYAMURAGIRA VOLCANIC ROCKS (VIRUNGA PROVINCE D.R. CONGO)   L.1, Morra V.1, Tedesco D.2, Balagizi Muhigirwa C.3 Minissale S.*1, Cucciniello C.1, Rocco I.1, Melluso

 

*[email protected]   1 Dipartimento di Scienze della Terra, dell'Ambiente e delle Risorse, Università di Napoli Federico II 2 Dipartimento di Scienze Ambientali, Seconda Università di Napoli 3 Département de Géochimie et Environnement, Observatoire Volcanologique de Goma, Nord-Kivu, RD Congo INTRODUCTION  AND  AIMS  

 

The   aim   of   this   study   is   the   characterizaCon,   through   petrographic   and   geochemical   analyses,   of   some  nephelinites  and  basanites  from  Nyiragongo  and  Nyamuragira  volcanoes  respecCvely  as  well   as  the  interpretaCon  of  the  magma  genesis  of  the  melilites  of  the  Nyiragongo  edifice.    

GEOLOGICAL  SETTING   The   Virunga   Volcanic   Province   (VVP)   lies   close   to   the   northern   end  of  the  western  branch  of  the   East   African   Ri^   System   (EARS).   Volcanism   started   about   11   Ma   ago   and   conCnuing   to   the   present.   (Demnant et al. 1994; Platz et al 2004).     T h e   t w o   a c C v e  

ANALYTICAL   METHODS      

• Petrographic   analysis;     • Whole-­‐rock   XRF;   • Trace   elements   ICPMS     • SEM-­‐EDS   microanalysis;    

volcanoes   of   VVP,   N y a m u r a g i r a   a n d   N y i r a g o n g o   a r e   located   along   the   s e i s m i c a l l y   a c C v e   sector  of  the  western   ri^.  (ChakrabarC  et  al.   2012).       Nyamuragira   (3058   a.s.l.)   is   a   large   volcanic   shield   characterized   by   alkaline   rocks   ranging   from   basanite   to   tephrite   and   rare   transiConal   basalt.  Nyiragongo  is  a  stratovolcano  (3469  a.s.l.)  characterized  by  rock   types   such   as   meliliCte,   melilite   nephelinite,   pyroxene   nephelinite,   leucite   nephelinite,   leuciCte   and   leucite   tephrite   (Andersen   et   al.,   2012;   Shama,1962).   Samples   include   parasiCc   cones   and   lava   fields   of   the   volcanic   complexes   from   1938   products   unCl   now,   and   products   of   2002   erupCon   sampled   from   the   proximal   vent   area   to   the   distal   outcrops.  

CLASSIFICATION  OF  THE  SAMPLES   Nyiragongo

16

phonolite

Nyamuragira

melilitites foidites

12

tephriphonolite

MINERALOGY  AND  GEOCHEMISTRY    

trachyte

10

phonotephrite

Monticellite

benmoreite

In  the  analyzed  samples,  olivine  ranges  from  forsterite-­‐fayalite  to  kirschsteinite;   clinopyroxene  is  mainly  diopside  and  melilite  has  akermanite  composiCon.  The   glass   composiCons   found  in   the   Nyiragongo   and   Nyamuragira  samples  have  a   wide   range   of   composiCons   (as   shown   by   TAS   diagram).   The   glass   in   the   Nyiragongo  rocks  o^en  is  rich  in  Ba  (up  to  5  wt%  BaO).  

Kirschsteinite

rhyolite

8

mugearite basanite tephrite

6

hawaiite

16

dacite

4

basaltic andesite andesite

basalt 2

0 35

40

45

50

55

60

65

70

75

80

Diopside

nepheline  

Hedenbergite

tephriphonolite

12

trachyte

phonotephrite melilitites benmoreite foidites basanite mugearite tephrite hawaiite andesite basaltic basalt andesite

10 8 6 4 2

PETROGRAPHY   The  basanites  and  tephrites  of  Nyamuragira   basanite  

rhyolite dacite

0 35

Augite

olivine  

Pigeonite Ferrosilite

Enstatite

clinopyroxene   1000

melili3te  

sample/chondrite

nephelinite  

Fayalite

Forsterite

SiO2  (wt  %)  

show  a  porphyriCc  texture  with  phenocrysts   of   olivine,   clinopyroxene   set   in   an   ipocristalline   groundmass   formed   by   plagioclase,   opaque   oxides,   nepheline   and   glass.   The   least   differenCated   mafic   rocks   of   Nyiragongo   are   melilite   nephelinites   with   primary   nepheline   +   leucite   +   melilite   +   clinopyroxene   associaCon;   and   olivine   meliliCtes   with   primary   melilite   +   olivine   +   clinopyroxene  +  nepheline  associaCon  

phonolite

14

Na2O+K2O  (wt.%)  

Na2O+K2O  (wt.%)  

14

Nyiragongo  

100

Nyamuragira  

10

melilite  

40

45

50

55

60

65

70

75

80

SiO2  (wt  %)   Nyamuragira   basanites   have   MgO   (12.05-­‐13.60   wt.%),   Cr   (790-­‐926   ppm)   and   Ni   (245-­‐309   ppm)   contents   within   the   ranges   expected   for   mantle-­‐derived   liquids.  The  transiConal  basalts  have  higher  MgO  (>  15  wt.%),  Cr  (>  969  ppm)   and  Ni  (>  750  ppm)  than  basanites.  Such  enrichment  in  these  elements  is  due   to  excess  of  olivine  phenocrysts.  Nyamuragira  basanites  have  Zr/Nb  (3.9-­‐4.0),   Ba/Nb  (11-­‐12)  and  La/Nb  (0.86-­‐0.9)  raCos  typical  of  mantle  or  OIB  values.  The   primiCve   mantle-­‐normalized   incompaCble   element   panerns   of   Nyamuragira   show   peaks   at   Ba   and   Nb   and   smoothly   decreasing   normalized-­‐abundances   from   Nb   to   Lu.   The   high   Lan/Ybn   raCo   (18)   indicates   that   the   Nyamuragira   basanites   are   low   degree   parCal   melts   of   a   slightly   incompaCble   element-­‐ enriched  mantle  source  in  the  garnet  stability.   The   least   differenCated   mafic   rocks   (olivine   meliliCte)   of   Nyiragongo     CaO   (~16.3   wt.%)   and   lower   SiO2   (~40   wt.%),   MgO   (8.7-­‐9.1   wt.%)   and   compaCble   elements   concentraCons   (Cr   =   380-­‐395   ppm;   Ni   =   155-­‐169   ppm)   than   Nyamuragira   basanites.   Their   incompaCble   element   panerns   are   also   more   enriched  than  those  of  Nyamuragira  basanites  with  higher  LREE/HREE  (La/Yb  =   42).   1000

1

nepheline  

olivine  

leucite   melilite  

leucite   melilite  

The  low  Zr/Nb  (2.1)  of  the  olivine  meliliCtes  indicate   that   the   Nyiragongo   olivine   meliliCtes   are   melt   products   of   an   incompaCble   element-­‐enriched   source.   In   addiCon,   their   low   heavy   REE   contents   suggest  that  they  were  generated  within  the  garnet-­‐ peridoCte   stability   field   within   the   lithospheric   mantle.    

nepheline  

sample/primitive mantle

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

100

10

Nyiragongo  

Nyamuragira  

1

Rb Ba U Th K Nb La Ce Pb Pr Sr Nd P Sm Zr Eu Ti GdTb Dy Ho Y ErTmYb Lu

olivine  

Olivine  melili3tes   CONCLUSIONS    

They   show   a   porphyric   texture   with   a   ipo-­‐hyaline   or   mycro-­‐ cristalline   groundmass.   All   samples  are  feldspar-­‐free.         REFERENCES  

Melilite   nephelinite  

The   Nyiragongo   and   Nyamuragira   volcanic   rocks   have   all   the   geochemical   features   expected   from   conCnental,   within-­‐plate   igneous   rocks.   They   are   interpreted   as   low-­‐volume,   low-­‐degree   melts   of   a   garnet   peridoCte.   The   composiConal   variaCon   within   the   Nyiragongo   volcanic   rocks   was   largely   controlled   by   low   pressure   fracConal   crystallizaCon   of   the   observed   phases.  The  source  of  the  Nyiragongo  and  Nyamuragira  rocks  is  inferred  to  be  the  lowermost  part  of  a  thinning  lithosphere,   which  melted  as  a  response  to  the  moderate  extensional  events  related  to  the  western  branch  of  East  African  Ri^  system.    

ANDERSEN  T.,  ELBURG  M.,  ERAMBERT  M.(2012)  Petrology  of  combeite-­‐  and  gozenite-­‐bearing  nephelinite  at  Nyiragongo,  Virunga  Volcanic  Province  in  the  East  African  Ri^.  Lithos  152  105-­‐121   CHAKRABARTI  R.BASU  AR.,  SANTO  AP.,  TEDESCO  D,  VASELLI  O.  (2009)  -­‐  Isotopic  and  geochemical  evidence  for  a  heterogeneous  mantle  plume  origin  of  the  Virunga  volcanics,  Western  ri^,  East African  Ri^  system.     Chem  Geol  259:273–289.   DEMANT  A,  LESTRADE  P,  LUBALA  RT,  KAMPUNZU  AB,  DURIEUX  J  (1994)  Volcanological  and  petrological  evoluCon  of  Nyiragongo  volcano,  Virunga  volcanic  field,  Zaire.  Bull  Volcanol  56(1):47–61 PLATZ  T,  FOLEY  SF,  ANDRE  L  (2004)  Low-­‐pressure  fracConaCon  of  the  Nyiragongo  volcanic  rocks,  Virunga  Province,  D.R.  Congo.  J  Volcanol  Geotherm  Res  136:269–295   SAHAMA  TG  (1962)  Petrology  of  Mt  Nyiragongo.  Trans  Edinburgh  Geol  Soc  19:1-­‐29