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Open-File Report No. Neigc16c:C3 DEPARTMENT OF AGRICULTURE, CONSERVATION AND FORESTRY Maine Geological Survey Robert G. Marvinney, State Geologist OPEN-FILE REPORT NO. NEIGC16C:C3 Title: Plutonic Rocks from Waldoboro to Richmond, Maine Authors: Amber T.H. Whittaker, Henry N. Berry IV, and David P. West, Jr. Date: October 2016 Reprinted in color, by permission, from: Berry, Henry N., IV, and West, David P., Jr., editors, 2016, Guidebook for field trips along the Maine coast from Maquoit Bay to Muscongus Bay: New England Intercollegiate Geological Conference, 108th Annual Meeting, October 14-16, 2016, Bath, Maine, 326 p. Contents: 28 p. report Recommended Citation: Whittaker, Amber T.H., Berry, Henry N., IV, and West, David P., Jr., 2016, Plutonic rocks from Waldoboro to Richmond, Maine: in Berry, Henry N., IV, and West, David P., Jr., editors, Guidebook for field trips along the Maine coast from Maquoit Bay to Muscongus Bay: New England Intercollegiate Geological Conference, p. 267-294. NEW ENGLAND INTERCOLLEGIATE GEOLOGICAL CONFERENCE 108th Annual Meeting Guidebook for Field Trips along the Maine Coast from Maquoit Bay to Muscongus Bay Edited by Henry N. Berry IV and David P. West, Jr. Hosted by The Maine Geological Survey and The Middlebury College Geology Department October 14-16, 2016 Copies of this guidebook, as long as they last, may be purchased for $25 from the following address: Geology Department Middlebury College 276 Bicentennial Way Middlebury, VT 05753 Cover Credit The cover photograph is by Arthur M. Hussey II, to whom this guidebook is dedicated. Arthur Hussey was an accomplished photographer and his numerous photo collections highlighted many aspects of the natural beauty of southwestern Maine. The photo was taken by Arthur at a location about a kilometer south of Lookout Point along the western shore of Harpswell Neck. Arthur first began mapping in this area in 1962, and his 1965 NEIGC field trip visited exposures nearby. The view in the photo is towards the south and the exposures are east-dipping metamorphosed Ordovician volcanic rocks of the Cushing Formation. Arthur's hammer for scale. The field guides for this Conference are offered under the terms of the Creative Commons Attribution-Non-Commercial 3.0 Unported DISCLAIMER Before visiting any of the sites described in New England Intercollegiate Geological Conference guidebooks, you must obtain permission from the current landowners. Landowners only granted permission to visit these sites to the organizers of the original trips for the designated dates of the conference. It is your responsibility to obtain permission for your visit. Be aware that this permission may not be granted. Especially when using older NEIGC guidebooks, note that locations may have changed drastically. Likewise, geological interpretations may differ from current understandings. Please respect any trip stops designated as "no hammers", "no collecting" or the like. Consider possible hazards and use appropriate caution and safety equipment. NEIGC and the hosts of these online guidebooks are not responsible for the use or misuse of the guidebooks. Printed by Reprographics, Middlebury College, Middlebury, VT 05753 This document is printed on recycled paper ii C3-1 PLUTONIC ROCKS FROM WALDOBORO TO RICHMOND, MAINE by Amber T. H. Whittaker, Maine Geological Survey, 93 State House Station, Augusta, Maine, 04333, [email protected] Henry N. Berry IV, Maine Geological Survey, 93 State House Station, Augusta, Maine, 04333, [email protected] David P. West, Jr., Middlebury College, McCardell Bicentennial Hall 428, Middlebury, Vermont, 05753, [email protected] INTRODUCTION This field trip will provide opportunities to visit representative exposures of several prominent Late Silurian to Devonian plutonic rock bodies exposed in the mid-coastal region of Maine. These stops will not only highlight the wide variety of intrusive rock types in the region, but it will also allow for discussions of their relationships to surrounding deformed metamorphic rocks and the constraints they provide on the geologic history of the region. Figure 1 provides a generalized geologic map of the region (with stop locations) and illustrates the spatial distribution of the major plutonic rock bodies and their relationships to the various lithotectonic belts they intrude. Importantly, although all the rocks visited on this trip were once pristine intrusive igneous rocks, overprinting deformational and metamorphic events have affected many of them and transformations into variably foliated to gneissic rock types are common. Although more complex, these rocks can not only be investigated in terms of their original igneous origins, but they also provide constraints on the nature and timing of superimposed tectonic events. The trip will proceed from east to west and will begin with stops in various intrusive rock bodies in and around the Waldoboro Pluton Complex (Sidle, 1990; Sidle, 1991; Sidle, 1993; Barton and Sidle, 1994). These will include the Raccoon gabbro to the east of the complex, the South Pond porphyry, and quartz diorite, garnet-bearing granite, and two-mica granite phases of the Waldoboro Pluton complex. The second half of the trip will provide opportunities to view strongly deformed rocks of the Lincoln syenite within the Haskell Hill granite gneiss, undeformed two-mica granite, multiple phases of the Blinn Hill plutonic complex, and finally the enigmatic Edgecomb gneiss. We have two general goals for this field trip, both of which we hope will provide benefits to a wide variety of experience levels. First, we will provide an opportunity to observe a wide variety of intrusive igneous rocks (granite to gabbro) that have been differentially affected by overprinting tectonic events (none to severe). In this context we will observe different mineralogies and textures (both primary igneous and overprinting) and discuss how these rocks can be classified on an individual basis. Secondly, we hope to place the various igneous rock bodies visited on this trip into a wider geologic context. Specifically, how the magma forming events fit temporally and spatially into the wider tectonic evolution of this region. CLASSIFICATION OF IGNEOUS ROCKS There are numerous methods used to describe and classify igneous rocks that range from very basic field identification using color, texture, and visible mineralogy, to extremely detailed classification by comparison of trace element composition obtained in the laboratory. The method of classification depends on the level of detail available. An initial observation of an igneous rock as light-colored or dark-colored starts the classification. Color can be misleading, as it depends on grain-size and weathering, however, a light-colored rock is assumed to contain high proportions of feldspar and quartz and is called felsic, and a dark-colored rock is assumed to contain minerals composed of magnesium and iron and is called mafic. Rocks with an in-between composition may be called intermediate. In addition, felsic rocks are less dense than mafic rocks, and this property can be observed in the field simply by hefting a hand-sample. Texture is also an important field observation. Very fine-grained igneous rocks are aphanitic and more coarse- grained rocks, where minerals are visible to the unaided eye, are phaneritic. Geologists commonly use texture to infer the cooling history of the magma, with fine-grained rocks indicative of fast cooling, and thus potentially 267 C3-2 WHITTAKER, BERRY, AND WEST Figure 1: Simplified regional geologic map with field trip stops. extrusive, and coarse-grained varieties indicative of slower cooling and thus intrusive. Textures with two dominant grain sizes, where coarser-grained minerals (phenocrysts) are embedded in a finer-grained matrix or groundmass, are called porphyritic, and this texture hints at a more complex crystallization history. Coarse-grained igneous rocks have the potential to display a wide range of textures too numerous to list here, several of which will be seen on this trip. Phaneritic (intrusive) igneous rocks are generally classified and named by their modal mineralogy; that is, the volume percentage of major minerals identified at the outcrop or seen in thin section. The relative proportions of quartz, alkali feldspar, plagioclase, feldspathoids, and mafic minerals, and the texture of the igneous rock (whether the rock is intrusive or extrusive), dictate which mineralogical classification diagram should be used. The International Union of Geological Sciences (IUGS) recommends using the double ternary QAPF diagram (Quartz – Alkali feldspar – Plagioclase – Feldspathoid) if the mafic minerals are less than 90% of the rock (M < 90%). If M > 90%, the mafic minerals are used to classify the rock utilizing other ternary diagrams (not presented here). When appropriate, rock types in this field trip have been classified using the QAP diagram (none of these rocks contain feldspathoids) as defined by Le Maitre and others (2002); results are shown in Figure 2 and discussed below with each stop. If bulk rock chemical compositions are available, igneous rocks may be classified by their chemistry. There are dozens of methods employed when analyzing the geochemistry of igneous rocks; we present only a few examples here. Chemical classification might be based on binary variation diagrams (also called Harker diagrams), where the weight percent of one elemental oxide is plotted against another, commonly silica. The TAS diagram uses the total alkali (Na2O + K2O) content
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