| Summary: | Andesite is one of mine commodity which the occurrence is abundant in
Pacitan (big potential), it is mostly mined and used as construction material. In
order to optimize on the usage of andesite as construction material and to
establish the appropriate method for exploration that will be worked, therefore
this research explain about petrogenesis of andesite, quality of andesite, and the
correlation among characteristic of petrology, geochemical and engineering
properties of andesite by using several methods. The methods are geological
mapping, petrography analysis, chemical analysis (XRF and ICP-MS),
engineering properties test (density, absorbed water, durability and compressive
strength of rocks), and statistical analysis (correlation and regression). Andesite
in Tanjungsari and surrounding area is belong to Arjosari Formation, above this
andesite, conglomerate unit, siltstone unit, and sandy clay sediment unit was
sedimented which bordered by uncorformity boundary. Andesite in research area
is product of Tertiary volcanic activity, formed as lava from andesitic magma with
57,59 � 59,14 wt % SiO2 content and alkalinity from tholeiitic to calc-alkaline.
This magma was formed in tectonic setting in subduction zone, precisely in calcalkaline
island arc area. Source of magma derived from pikritic magma or olivine
basalt from N-MORB mantle which experienced Rb, Ba, Th, and K enrichment
from subducted crust, and through fractional crystallization process of olivine
and clinopiroxene so caused magma composition changing into andesitic. The
results of engineering properties test show that compressive strength of andesite
in research area is categorized into weak, strong enough (moderately strong) and
strong, and generally the quality of andesite is appropriate to standard of nature
rocks for construction materials. The results of correlation and regression
analysis give the trend of relation among character of petrology, geochemical and
engineering properties of andesite, and give regression model.
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