TITLE:
Geochemistry and Petrology of Basic Volcanic Rocks of Jabal Al Haruj Al-Aswad, Libya
AUTHORS:
N. M. Al-Hafdh, Abd El-Salam S. El-Shaafi
KEYWORDS:
Jabal Al Haruj Al Aswad Area, Basaltic Rocks, Geochemistry, Petrology, Petrogenesis, Libya
JOURNAL NAME:
International Journal of Geosciences,
Vol.6 No.1,
January
30,
2015
ABSTRACT: The Al Haruj
Intra-continental Volcanic Province is the largest part of the extensive
volcanic activity in Libya which is considered to be a typical within plate
basalts. The volcano-tectonics evolution of this province, as well as its
origin, are still widely disputed. According to K-Ar dating previously studied,
the volcanic activity started in the Late Miocene and lasted until at least the
Late Pleistocene. The field may still be volcanically active. The mafic rocks
of Jabal Al Haruj have been classified into six major phases or groups. These
phases have been differentiated using Landsat images together with aerial
photographs of different scales as well as field observations. The topographic
forms of the earliest phase are highly eroded while the forms of the latest
phase are usually fresh and very well preserved as regards primary features.
Mafic lavas of this field consist of alkali basalts to olivine tholeiites
(transitional basalt) which contain olivine as essential constituent together
with clinopyroxene, plagioclase and glass. The basalt exhibits intergranular,
intersertal, ophitic and subophitic relations. Amygdaloidal and
glomerporphyritic textures are also observed. The basaltic rocks of different
ages and from different localities are petrographically rather similar.
Phenocrysts of olivine probably the result from slow cooling in crustal magma
chambers prior to eruptions, suggesting that magmas ascended slowly through the
crust. 109 samples have been carefully collected from various phases, some of
these samples have been chosen for major and trace elements analyses, using XRF
in order to determine the characteristics of themantle source and investigate crustal
interaction. The major and trace elements revealed aslightly significant chemical
diversity among the phases and within each phase. The normative classification of
most of these rocks shows close agreement with their modal classification. A
vague correlation between MgO and most major oxides in the studied samples
suggests different degrees of partial melting rather than fractional
crystallization. A characteristic feature of the studied volcanic rocks is the
relatively constant ratios of certain incompatible trace elements (Nb/Zr,
Rb/Zr), which provides strong evidence of a common source. In addition, the
rocks display similar patterns of the peaks and troughs; this strongly suggests
that they have a common parent and common subsequent processes. The compatible
transitional metals Ni (81 - 193 ppm) and Cr contents (238 - 361 ppm) and
relatively low Mg# (Mg/(Mg + Fet)) (52 - 62) give an indication that the
studied basaltic rocks have slightly to moderately fractionated olivine and/or
spinel. The magmatism of this volcanic field seems to be related to
reactivation of pre-existing structures during the passive rifting of the Sirt
Basin that most likely produced in response to convergence between European and
African plates since Jurassic until Holocene times.