The Highland Border Ophiolite (HBO) is a set of rocks that are

ophiolitic An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks. The Greek word ὄφις, ''ophis'' (''snake'') is found in the name of o ...

in character found within the Highland Border Complex. They are exposed in a series of fault-bounded outcrops along the line of the

Highland Boundary Fault The Highland Boundary Fault is a major fault zone that traverses Scotland from Arran and Helensburgh on the west coast to Stonehaven in the east. It separates two different geological terranes which give rise to two distinct physiographic ter ...

that forms the southeastern boundary to the Grampian Highlands in Scotland. They represent fragments of a piece of

oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramaf ...

or exhumed mantle that has been obducted onto

continental crust Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as '' continental shelves''. This layer is sometimes called '' si ...

. These rocks provide an important constraint on models of how the current geometry arose during the

Caledonian Orogeny The Caledonian orogeny was a mountain-building cycle recorded in the northern parts of the British Isles, the Scandinavian Caledonides, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events tha ...

Lithology

The main lithologies found within the HBO are

serpentinite Serpentinite is a metamorphic rock composed predominantly of serpentine group minerals formed by serpentinization of mafic or ultramafic rocks. The ancient origin of the name is uncertain; it may be from the similarity of its texture or color ...

, often in the form of ophicarbonate, with associated

sedimentary rock Sedimentary rocks are types of rock (geology), rock formed by the cementation (geology), cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or de ...

s, including sandstone, limestone, chert and black mudstone (now black

slate Slate is a fine-grained, foliated, homogeneous, metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash through low-grade, regional metamorphism. It is the finest-grained foliated metamorphic ro ...

). The serpentinite is interpreted to be derived from exhumed mantle

peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high pr ...

s, altered to serpentinite at the seafloor. This is consistent with the presence of ophicarbonates that are associated with serpentinization in such an environment.

Interpretation

The parts of the Highland Border Complex that are not ophiolitic have been assigned to the Trossachs Group, interpreted to be the youngest part of the Dalradian Supergroup, although this interpretation has been questioned. The Trossachs Group as interpreted ranges in age from

Lower Cambrian The Cambrian ( ) is the first geological period of the Paleozoic Era, and the Phanerozoic Eon. The Cambrian lasted 51.95 million years from the end of the preceding Ediacaran period 538.8 Ma (million years ago) to the beginning of the Ordovici ...

Lower Ordovician The Early Ordovician is the first epoch of the Ordovician period, corresponding to the Lower Ordovician series of the Ordovician system. It began after the Age 10 of the Furongian epoch of the Cambrian and lasted from to million years ago, unt ...

, based on

trilobite Trilobites (; meaning "three-lobed entities") are extinction, extinct marine arthropods that form the class (biology), class Trilobita. One of the earliest groups of arthropods to appear in the fossil record, trilobites were among the most succ ...

fossils found in the Leny Limestone and

conodont Conodonts, are an extinct group of marine jawless vertebrates belonging to the class Conodonta (from Ancient Greek κῶνος (''kōnos''), meaning " cone", and ὀδούς (''odoús''), meaning "tooth"). They are primarily known from their hard ...

fossils found in the Margie Limestone. There is evidence that the HBO was thrust on top of the Trossachs Group before the first phase of the Caledonian Orogeny (the Grampian phase) affected either of these units later in the Ordovician. In this model, the HBO represents pieces of exhumed mantle from the non-volcanic margin of Laurentia, transported onto the Dalradian Supergroup with its sedimentary cover. The HBO has been correlated with the Deerpark Complex of western Ireland, which consists of fault-bounded lenses of serpentinite, juxtaposed with

amphibolite Amphibolite () is a metamorphic rock that contains amphibole, especially hornblende and actinolite, as well as plagioclase feldspar, but with little or no quartz. It is typically dark-colored and dense, with a weakly foliated or schistose ...

s and

metasediment In geology, metasedimentary rock is a type of metamorphic rock. Such a rock was first formed through the deposition and solidification of sediment Sediment is a solid material that is transported to a new location where it is deposited. It occu ...

s. It is faulted against the Clew Bay Complex, which has been correlated with the Highland Border Complex.

References

{{Reflist Geology of Scotland Highland Boundary Fault Ophiolites