Page 13 - Volcanic Textures A Guide To The Interpretation of Textures In Volcanic Rocks

Part 1. Interpreting textures: terminology and

techniques

The important task of exploring for base metal ore emplacement or deposition. Their original textures are
deposits in ancient submarine volcanic sequences begins almost invariably modified by processes related to
with geological mapping, which in turn depends on cooling. All types of volcanic and volcaniclastic
correct identification of outcrops and hand specimens. deposits, particularly those that initially contained
The text and illustrations that follow are designed to volcanic glass, can be subject to post-volcanic textural
help meet this significant challenge, by providing a modification.
guide to the interpretation of common textures and
structures in volcanic deposits. Volcanic deposits Emphasis and organization
include consolidated volcaniclastic and coherent
volcanic rocks, and unconsolidated volcaniclastic This guide emphasises the principal genetic processes
aggregates. The examples used are principally from the responsible for the creation of original textures, and the
Mount Read Volcanics in western Tasmania, a most common syn-volcanic modifications. The
deformed, Cambrian, largely submarine volcanic organization of the plates and text reflects a process-
sequence that hosts major massive sulfide deposits oriented classification (Fig. 1) that also includes
(Large, 1992). We have also included, for comparison, volcaniclastic deposits generated by resedimentation
examples from other well-exposed and younger, and by purely sedimentary processes. The classification
submarine volcanic sequences, and from subaerial shows the main transport and deposition mechanisms
volcanic sequences. involved in the formation of pyroclastic, resedimented
volcaniclastic and volcanogenic sedimentary deposits. It
The imprint of genetic processes on is very important to appreciate that similar mechanisms
operate in each of these categories and, hence, similar
textures in volcanic deposits textures and structures may result.
The development of textures in volcanic deposits can be
considered in terms of three main stages: Components, textures and structures (Part 2) are
(1) creation of original textures by eruption and inherent properties of volcanic deposits and provide the
emplacement processes; basis for descriptive nomenclature. Some give tight
(2) modification of original textures by syn-volcanic constraints on genetic interpretations but many merely
processes (oxidation, degassing, hydration, vapour- suggest a number of equally valid alternatives. The
phase alteration, high-temperature devitrification, selection of components, textures and structures in Part
hydrothermal alteration); 2 is not comprehensive but instead concentrates on
(3) modification by post-volcanic processes (hydration, features that help distinguish volcaniclastic from
devitrification, hydrothermal alteration, diagenesis, coherent volcanic deposits, that survive in ancient
metamorphism, deformation, weathering). sequences and can be recognized in outcrop or hand
specimen with a hand lens. Part 3 of the guide describes
Of overriding importance in the creation of original structures and facies associations in the products of
textures in primary volcanic deposits is the eruption effusive eruptions (lava flows and lava domes) and syn-
style, in particular whether explosive or effusive (Fig. volcanic, high-level intrusions (sills, dykes,
1). Explosive eruptions produce a wide variety of cryptodomes). These are typically associated with
pyroclastic deposits. Effusive eruptions produce lava autoclastic deposits that are generated by non-explosive
flows and lava domes that comprise coherent and fragmentation (quenching, autobrecciation). Part 4 deals
autoclastic facies. A third category of "volcanic" with volcaniclastic deposits produced by explosive
deposits is created by the syn-volcanic emplacement of eruptions (primary pyroclastic deposits), their syn-
cryptodomes, dykes and sills, and also includes coherent eruptive resedimented equivalents, and volcaniclastic
and autoclastic facies. In active volcanic terranes, deposits resulting from surface processes operating on
whether subaerial or subaqueous, diverse volcaniclastic pre-existing volcanic sequences (volcanogenic
deposits are generated by non-volcanic processes, sedimentary deposits). Correct interpretation of these
specifically by resedimentation coeval with or deposits relies on recognition of (1) structures and
independent of eruptions and by weathering, erosion lithofacies characteristics that indicate transport and
and reworking of pre-existing volcanic deposits. These depositional processes; (2) textures and structures that
are sometimes referred to as secondary volcaniclastic indicate whether or not the particles were hot when
deposits, whereas undisturbed pyroclastic and deposited; (3) textures of constituent particles that
autoclastic deposits are primary. Mechanisms of particle indicate clast-forming processes. Finally, in Part 5, the
transport and deposition are of great importance in the alteration textures in volcanic sequences that host
creation of original textures and structures in massive sulfide deposits are reviewed, using examples
pyroclastic, resedimented and volcanogenic sedimentary from the Mount Read Volcanics. An important aim of
deposits. Lavas, syn-volcanic intrusions and many types this part is to show that unraveling the complex web of
of primary pyroclastic deposits are hot at the time of interrelated processes involved in alteration of volcanic

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