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