The
geochemical characteristics of the
boninites
of
Eastern (SSZ) Albanian ophiolites
ABSTRACT
By
*Dr.Kristina
Gjeçi*, **Msc.Sander Kovaci**
*Institute
of Geosciences, Polytechnic University
of Tirana (UPT), ALBANIA.
Email:mirdita82@yahoo.com
**Faculty
of Mathematical and Physical Engineering,
Dept. of Mathematical Engineering,
Polytechnic University of Tirana (UPT),
ALBANIA.
In the
Albanian ophiolites, boninites are found
within Eastern (SSZ) – type ophiolites
as part of their volcanic sequence, and
sheeted dike complex; in the plutonic
gabbro–plagiogranitic sequence and very
rare in ultramafic intrusions and in
uppermost part of ultramafic sequence.
They are porphyric, microporphyric,
vitroporphyric with CPx –endiopsid(aught)
chromitic with( Wo 38.5-43.58; en 50.98
– 53.37; fs 5.12 – 8.69);Cr2O3
> 0.15% and OPx -
enstatite-bronzite (Wo -18.12;
en-80.61;fs-3.27), olivine phenocrysts
and accessory cromspinelide.Absent
plagioclase phenochrits.
Microphenocrysts are Opx rare Cpx and
chromshpinelid .The groundmass is small
grain, microlitic (Opx, Cpx, Pl),
sometimes chlorite and obsidian (volcanic
glass) with basic and average contain.
They rare meet fresh, are partially or
totally alterated.
Chemical
composition of boninites shows very low
(< 0.45%) TiO2 content, low (average
0.5) CaO/Al2O3
ratio, high (> 7.5%) MgO content and
low ETR content. Boninites of the
Eastern (SSZ) –type ophiolite are mainly
low –Ca boninites, rarely high-Ca
boninites according to Crawford et al.
(1989) classification.
Based in
chemical dates and petrographic
charachterstics boninites of Albanian
Eastern (SSZ) - type ophiolites
correlation with another boninites
suite by world, Cape Vogel, Chichi Jima,
Mariana Trench, Baja California and
Cyprus. Boninites of Albanian Eastern (SSZ)
- type ophiolites are mainly low - Ca
boninites and rarely high – Ca boninites
according to Crawford et al.(1989)
classification bat these boninites are
differently on contain mineral and
petrographic charachterstics ,for
example: the boninites of Albanian
Eastern(SSZ) - type ophiolites (low -
Ca boninites) are more poor in SiO2 ,
Na2O,K2O and moor rich on Al2O3 as Cape
Vogel,Chichi Jima , Mariana Trench, Baja
California(tab.1). The boninites of
Albanian Eastern (SSZ) - type ophiolites
(high – Ca boninites) are richer on
SiO2, Na2O.K2O and moor poor in CaO as
boninites Cyprus (tab.1).
The
boninites of Albanian Eastern (SSZ) -
type ophiolites correlation with
boninite suites Cape Vogel, Chichi Jima,
Mariana Trench, Baja California and
Cyprus in contain LREE and ETR ( tab.2)
Boninites
are associated with high-MgO and
low-TiO2 basalts, and with
andesitedacites and rhyodacites with
boninitic affinity, forming a
Boninitic
association, which together with
predominating IAT
basalt-andesite-rhyodacite constitute an
association similar to the Western
Pacific arc lavas generated at fore arc
settings.
Formation
of the boninites of Eastern-type
ophiolite is caused by partial melting
processes of the refractary mantle
peridotite and mantle-melt interaction.
ETR.
Content for boninite from central
Mirdita (Albania) and from suites Chichi
Jima, Mariana Trench, Cape Vogel, as
Gjeçi K. (2008) based on the Hickey and
Frey (1982) and Dilek Y., Furness
H., Shallo M.
(2007).
Tab.2
|
|
Chichi Jima
2981 |
Mariana Trench
2980 |
Cape Vogel
2984 |
Bon. Central Mirdita
90
Al-00
|
|
La |
1.27 |
1.27 |
1.47 |
1.10 |
|
Ce |
2.57 |
2.93 |
3.51 |
1.3 |
|
Nd |
1.65 |
1.85 |
2.09 |
0.93 |
|
Sm |
0.426 |
0.508 |
0.602 |
0.27 |
|
Eu |
0.146 |
0.171 |
0.210 |
0.01 |
|
Tb |
0.099 |
0.115 |
0.144 |
0.07 |
|
Yb |
0.591 |
0.602 |
0.809 |
0.55 |
|
Lu |
0.103 |
0.104 |
0.133 |
0.1 |
The
average content of indicators elements(major)
for the boninite of Eastern (SSZ)
Albanian ophiolites and for the boninite
of World ophiolites,as Gjeçi.K(2008)
based on the (Gjeçi K.,Shallo M.(2006);Shallo
M. (1994,1995); Gjata,Beccaluva(1994
Milushi I.etc.(2002);
Manika K. (1994); Manika K., Shallo
M.,Gega D. (1999).
Tab.1
|
|
Bon.
Volc.
Sequ
ence
|
Bon.
Shee
ted
dike complex
|
Bon.
plutonic gabbro–plagiog.
sequence
|
Bon.
Ultra
mafic
intru
sions
|
Bon.
New
Cale
donia
suites |
Bon
Mariana Trench
suites |
Bon.
Seto
uchi
suites |
Bon
Baja-
Kalifo
- rnia
suites
|
Bon.
Chichi
-Jima
suites |
Bon.
Cape
Vogel
suites |
Bon.
Cyprus |
Bon. Tonga Trench suites |
|
SiO2 |
52.26 |
52.63 |
54.15 |
52.10 |
57.08 |
58.24 |
57.74 |
56.50 |
56.20 |
54.06 |
50.75 |
54.41 |
|
TiO2 |
0.31 |
0.23 |
0.13 |
0.21 |
0.26 |
0.17 |
0.51 |
0.69 |
0.13 |
0.21 |
0.23 |
0.33 |
|
MgO |
9.26 |
11.58 |
17.83 |
11.72 |
12.58 |
13.25 |
8.61 |
8.0 |
11.19 |
14.57 |
11.0 |
14.84 |
|
CaO/
Al2O3 |
0.64 |
0.70 |
0.63 |
0.65 |
0.63 |
0.43 |
0.45 |
0.42 |
0.70 |
0.66 |
0.85 |
0.87 |
|
Mg# |
0.51 |
0.55 |
0.86 |
0.50 |
0.38 |
0.63 |
0.57 |
0.58 |
0.57 |
0.61 |
0.57 |
0.62 |
|
Na2O |
2.06 |
1.28 |
0.12 |
0.94 |
2.29 |
2.07 |
2.74 |
4.1 |
1.54 |
0.92 |
0.9 |
1.19 |
|
K2O |
0.28 |
0.26 |
0.00 |
0.18 |
0.49 |
0.81 |
1.81 |
1.03 |
0.40 |
0.30 |
0.22 |
0.29 |
|
Alcaline
total |
2.34 |
1.54 |
0.12 |
1.12 |
2.78 |
2.88 |
4.55 |
5.13 |
1.94 |
1.22 |
1.12 |
1.48 |
|
Boninites |
of
|
Eastern
(SSZ) |
Abanian
ophilites |
|
Bon. |
of |
World |
ophilites |
|
|
|
|
Reference:
Crawford A. J., Fallon T. J. and Green
D.H. (1989)
–
Classificaton, petrogenesis and tectonic
settings of boninites.
Dilek
Y., Furness H., Shallo M. (2007) –
Suprasubduction zone ophiolite formation
along the periphery of Mesozoic
Gondwana. Gondwana research 2007.
Hickey
R., Frey F.A. (1982)
- Geochemical characteristisc of
boninite series volcanic Implications
for their source. Geochim. Cosmochim.
Acts 46, p. 2009 – 2115.
GJeçi K., Shallo M., (2006)
-Vendosja gjeologjike e boniniteve ne
ofiolitet e tipit lindor te Shqiperise
dhe veçorite petrografike e
gjeokimike te tyre. Buletini i shkencave
gjeologjike, 1/ 2006, 3-16.
Gjeçi K.(.2008) -
Petrologjia e boniniteve te ofioliteve
lindore te Shqiperise (PhD material).