THIN SECTION DESCRIPTIONS OF INTRUSIVE AND METAMORPHIC ROCKS FROM THE CRESWICK 1:100000 SHEET - Geological Survey of Victoria Unpublished Report No. 1998/11

Sep 20, 2026

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Unpublished Geological Survey of Victoria report detailing thin section descriptions for intrusive and metamorphic rocks from the Creswick 1:100000 map sheet (Vicmap no. 7623). Specimens are largely from mine dumps, with context limited where rocks are overlain by Newer Volcanics and alluvium. The document organizes specimens by intrusive rock types, listing GSV thin section catalogue numbers with field numbers and providing mineral assemblages, alteration phases, texture, and structural features for each sample. Includes descriptions of quartz diorite and tonalite-related porphyries.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages24

Reading Time1 h 0 min

AccessFree to download

Introduction

Intrusive rocks

Sample 26198 (LH9): Quartz diorite with enclaves - Homebush Granite

Sample 26055 (1-183): Clover Hill Porphyry (altered tonalite)

Sample 26056 (1-182): Creswick Plug (trachyte)

Sample 26060 (2-267): Sorensens Track Porphyry (quartz diorite)

Geological Survey of VictoriaUnpublished Report No.1998/11 # THIN SECTION DESCRIPTIONS OF INTRUSIVE AND METAMORPHIC ROCKS FROMTHE CRESWICK1:100000 SHEET Vincent J.Morand August 1998 # INTRODUCTION The following thin sections were made from rocks collected mainly by Vince Morand,and come fromthe western part of the Creswick 1:100000 map sheet (Vicmap no.7623).This was part of the projectdetailing the geology and mineral explor.ion potential of the Creswick sheet.Many specimens arefrom mine dumps so their geological coniext is not well known,but these nevertheless provideimportant information on rocks that are covered by Newer Volcanics and alluvium. The rocks are divided into intrusive rocks and metamorphic rocks. The GSV thin section catalogue number is listed first,followed by the field number in parentheses. INTRUSIVE ROCKS 26198(LH9)Quartz diorite with enclaves-Homebush Granite(mine dump) AMG 724300-5898900 Host rock: Igneous minerals:hornbiende(pale green),plagioclase,orthopyroxene,biotite,quartz,pyrrhotite.Hornblende-rich. Texture:Coarse grained.Large hornblende grains have small rounded inclusions of orthopyroxene andplagioclase. Enclave type I (diorite): Igneous minerals:plagioclase,biotite(or phlogopite),spinel(green),hornblende(pale green),quartz,opaques.Plagioclase-rich. Alteration:clinozoisite,sphene Texture:Coarse grained,equigranular,peppered with very small anhedral spinels.Biotite is commonlyinterstitial.One of these enclaves is medium grained with a rim of fine grained plagioclase whichis partly aligned to form a trachytic texture -possibly a result of chilling of the enclave magmaagainst the host magma. Enclave type 2(?anorthosite): Igneous minerals:plagioclase(comprising about 90%of the rock),spinel(green),minor phlogopite andopaques Texture:Fine grained equigranular,but the spinels are much larger than in type 1 enclaves.Possibly arecrystallised igneous texture,although the rock cold also be a granulite facies metamorphicrock. The enclaves constitute more than 50%of the rock,and in hand specimen they are pancake shaped andaligned to form a planar fabric,possibly flow induced. 26055(1-183)Clover Hill Porphyry(altered tonalite) AMG 756500-5867300 Igneous minerals:plagioclase(most is altered to albite,but some has weak normal zoning),quartz(typically in micrographic intergrowth with altered/weathered feldspar),hornblende(brown toolive green,mostly altered),?biotite(completely altered),opaques(many altered to fine-grainedsphene or leucoxene,some with square shapes,some with hexagonal shapes-?hematite,somewith skeletal alteration pattern-ilmenite),zircon,apatite.The rock is hornblende-rich. Alteration:Plagioclase is altered to albite,sericite,prehnite(small plates and radiating clusters),epidote and chlorite.Hornblende is altered to chlorite,epidote,sphene and rare calcite.Biotite isaltered to chlorite,epidote and sphene;a relic cleavage is outlined by aligned epidote andsphene.Ilmenite is altered to leucoxene.Some large patches of calcite occur intergrown withquartz.Prehnite is most common in plagioclase. Texture:Largely equigranular,medium grained with a few prisms of hornblende forming phenocrystsup to 2 mm.Plagioclase is euhedral,quartz and most hornblende and biotite are anhedral.Micrographic intergrowth of quartz and feldspar is common.Hornblendes probably originallyformed as clusters of several grains. Structure:The rock is massive.Quartz shows little deformation or recrystallisation. The rock was originally a hornblende-biotite tonalite(I-type)and has been strongly altered undersubgreenschist facies conditions.Micrographic texture indicates a high level of intrusion. 26056(1-182)Creswick Plug(trachyte) AMG 760500-5853500 Igneous minerals:sanidine,anorthoclase,opaques(including hematite),red-brown amphibole-probably kaersutite(typically completely or marginally replaced by small magnetite grains). 4 Sanidine comprises about 80%of the rock,anor

FAQs

What thin section catalog information is recorded for each rock sample?

Each entry lists the GSV thin section catalogue number first, followed by the field number in parentheses, then mineralogy, alteration, texture, and structure.

Why is the geological context sometimes poorly known for the specimens?

Many specimens are collected from mine dumps, so their original geological context is not well known even though they still provide important information.

How are intrusive rocks organized in the report?

The report divides rocks into intrusive and metamorphic categories, and within intrusive rocks it presents multiple samples with separate descriptions for quartz diorite, altered tonalite porphyry, trachyte, and quartz diorite porphyry.

Sep 20, 2026

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