%D 2016
%X Abstract

Marble and Phyllite are well known as Pre-Tertiary basement rock crop out in Joko Tuo area, East Jiwo Hill, Central Java. These rocks contain fossil record even though they have undergone metamorphism. Preliminary petrographic study in several samples of show the existence of larger foraminifera Orbitolina sp. in good preservation condition between foliated of chlorite-biotit-graphite. Based on petrographic data, the metamorphic rocks that were studied comprised of foliated marble that dominantly consist of calcite, chlorite, biotit, graphite and feldspar. The protolith of metamorphic rock interpreted derived from fine grained calcareous sedimentary rock which was deposited in shalow marine environment at Late Cretaceous age.

Introduction

The protolith of a metamorphic rock is defined as the original rock prior to metamorphism. This is comprises rocks of all possible chemical compositions and include the entire range of sedimentary, igneous and metamorphic rocks. The protolith of a metamorphic rock can be interpreted based on the mineralogy and/or the rock texture.

The marble and phyllite in Joko Tuo area, East Jiwo Hills is interesting object for protolith study. Preliminary investigation show that these rocks contain fossil record even though they have undergone metamorphism. The preservation of fossils in metamorphic rocks is unusual, except in low-grade metamorphosed sedimentary rocks. This paper explains petrographical observation of some sample from Joko Tuo area. The study of protolith and surviving fossil content in this area will contribute the paleoenvironment and age of the Jiwo Hills terrane.

Geological Outline 

Geologically, Jiwo Hills, Bayat, Central Java (Figure 1) built by elevated basement rocks covered by a relatively thin marine sedimentary rock layers and intruded by numerous igneous rock bodies (Salahuddin & Novian, 2014). The basement complex mostly composed of metamorphic rocks, such as phyllites, mica schists, calc-silicate schist and marbles (Setiawan et al., 2013). Amphibolite, blue schist, and sepentinite also reported present in this area (Warmada et al., 2008). Among them, phyllites are the most widespread, generally in weathered condition with some quartz veins and calcite veins aligned with their foliation. In Pre-Tertiary Jiwo, no fossiliferous beds have been found, only boulders of Orbitolina limestone derived from neogene conglomerate have been found near desa Santren which shows that at least part of pre Tertiary sub stratum is Cretaceous age (Bothé, 1929). The age of this basement rocks by applying K-Ar absolute dating indicate an age of 98 mya or Late Cretaceous (Prasetyadi, 2007). This metamorphic rocks is interpreted to be formed during a regional metamorphism that involved along an ancient subducting lithospheric plate and the plates collision afterward (Setiawan et al., 2014). 


 
Figure 1:  Geological Map of Jiwo Hills (after Rahardjo, 1994). The red box is the location of Joko Tuo area.

Corresponding author : 
Herning Dyah Kusuma Wijayanti
herningdkusuma@gmail.com
%P 1-4
%L ITNYREPOID1080
%C The Trans Luxury Hotel, Bandung
%A . Herning Dyah Kusuma Wijayanti
%A . Okki Verdiansyah
%A . Moch. Indra Novian
%A . Nugroho Iman Setiawan
%A . Kholilur Rohman
%T Protolith of  Joko Tuo Marble, Bayat, Central Java; contribution to paleoenvironment and age of metamorphic rock.