Introductory Chapter: Introduction to New Advances in Metamorphic Geology

Abstract

Metamorphic geology is one of the oldest arms of geology and provided fundamental new insights to understand continental growth, terrain accretion, orogenesis and the principal of rock cycle. It is also one of the most diverse still remarkable focused subject area of geology that reflected in a relatively low number of specialized science magazines dedicated to metamorphic geology. In the past decades geology subjects based on more traditional, field-based approaches suffered from the sudden emergence of advanced technologies, modelling and experimental geology in the whole earth science. This trend seems to be changing these days due to the successful adaptation of those new advances to blend them with traditional geological techniques. Metamorphic geology is one of the subject areas where these changes are very visible and rapidly evolving. In this chapter a brief summary is provided to outline the major advances and emergences of new methods and techniques withing metamorphic geology creating a new renaissance of this important area of geosciences.

1. Introduction

1.1 Metamorphic geology as the core of traditional field geology

Metamorphism in the context of geology (https://opengeology.org/petrology/9-intro-to-metamorphism/ – accessed on 25 February 2024) is a process that changes the mineralogy, microstructure, texture, and chemical compositions of any rocks, normally over long time scale (millions of years) and in a dominantly solid state [1]. Fluid-driven transition of rock textures and compositions are commonly viewed as a special part of metamorphism (or not even included at all within metamorphism) normally associated with some fluid migration controlled by magmatism such as metasomatism on a sea floor in small or lithospheric scales, including ophiolites [2, 3, 4, 5, 6, 7]. In general, metamorphism is understood as a process that forms rock assemblages directly in response to gradually changing physical and chemical conditions. Metamorphism can be considered to begin when the original rock (protolith) faces new and persistent physical or chemical conditions markedly different from its original environment, or better to say the difference can be considered in a significant scale (https://geologyglasgow.org.uk/minerals-rocks-fossils/metamorphic-rocks/ - accessed on 25 February 2024). The classical view on metamorphism is based on the physical-chemical parameters to define the metamorphic environment including the temperature and pressure being the most important ones (https://www.geolsoc.org.uk/ks3/gsl/education/resources/rockcycle/page3576.html – accessed on 25 February 2024). In contrast, fluid availability and nature as well as the nature of the tectonic stress field are other aspects to view metamorphism [8, 9]. These categories almost automatically determine the main approaches metamorphic geology follow. Most importantly and traditionally, the most abundant method is the chemical characterization of the processes (e.g., petrography and petrology), following geochemical methods such as bulk chemical study of the metamorphic products or mineral-level documentation of the chemical (and textural) changes [10]. These methods are the most obvious ones one can think of about metamorphic geology.

2. Advances in scale to track steps of metamorphism

The main advances in this field are centered around the application of new chemical systematics to follow mineral phase changes and link them to a geological process. High-resolution microchemistry is commonly performed to lower the potential resolution of time scales of the metamorphic events recognizable; hence, a strong drive is visible for developing new techniques to determine the ages of various metamorphic events in the micromineralogy level [11, 12, 13, 14]. As metamorphism may occur over a wide range of temperature and pressure conditions, the applicable chemical techniques to define these conditions have formed a great variety of research avenues over the past decades. In this perspective, there is a clear and gradual transition from experts focusing on the primary rocks such as sedimentary successions specialized to recognize the early stages of solid phase transition of the original mineral constitutes of sediments and sedimentary rocks to experts clearly dealing with the metamorphic products from the sedimentary protoliths [15, 16, 17, 18, 19, 20]. It is also a common approach that the metamorphism starts when the diagenesis completed. While this sounds a trivia, to “capture” the distinct line between diagenesis and metamorphism is not simple. This fuzzy boundary between diagenesis and metamorphism naturally generated various schools following different distinctive parameters since the early 70s in the dawn of metamorphic geology. As in regional sense, these boundaries follow some temperature-pressure conditions, and it is not a surprise that the boundaries are also not fixed as they are also heavily dependent on the actual region geothermal gradients; hence, the limits are different in regions with high geothermal gradients such as orogenic belts [21, 22, 23].

3. Temperature-pressure concept of metamorphism

In the other end of the temperature-pressure spectrum, a similar problem exists as great variations known among suggested barriers. In extreme high temperature and/or pressure, the rock starts to behave as melt and shows similar features expected with igneous rocks. Melting itself in this context is not considered to be a metamorphic process, and we are entering to the unsharp boundary between metamorphic and magmatic processes. However, as melting considered in a rock when the temperature exceeds its wet solidus that is normally around 650°C for most metasediments and metabasites, the beginning of melting does not mean the end of metamorphism. There is a stage when potential melting through the metamorphic grades can take place, but it is hardly ever occurred in en mass and large scale; instead, following stages and pulses is likely associated with some external forcing mechanism as tectonism. Some research groups hence favor to delineate a fine line between metamorphic melting and igneous melting. The extracted melts form purely igneous intrusions within the crust normally shallower than the melting anomaly. In contrast, in situ restites that are the solid mineral assemblages after melt segregation may form gneisses or granulites of high metamorphic grade, or form migmatites [24] together with the locally segregated melts [25, 26]. Thermobarometer revealed that igneous intrusions at shallow crustal level indicate much lower temperatures from high-grade metamorphic rocks in the source region. The highest measured granulite formation temperatures suggest about 1000–1100°C considered to be the highest metamorphic temperatures [27]. It is obvious that metamorphic geology advanced significantly in recent years to refine these boundaries, or better to say look at metamorphic processes withing the broader geological context without implying the need to follow strict boundaries among processes. To explore the spectrum of pressure conditions within metamorphic processes opened new research directions and recognition of characteristic diagnostic minerals to look for. Ultra-high pressures of metamorphism [28, 29] have been recognized from rocks suspected to be subducted over the depth of 300 km followed by rapid uplift and preservation. These findings determine also new research directions to locate rocks that can be viewed as messengers of extreme and long sustained conditions.

4. Texture recognition techniques from micro to landscape scale

New techniques and modeling are also emerged just as more refined textural analysis of metamorphic fabric. The new mineral phases generated by metamorphic processes on the expenses of original minerals generate typical metamorphic textures extensively studied. The recognition of major textural changes is characteristic for the major physical-chemical condition changes. This means that, in metamorphic geology, the recognition of rock textures, such as metamorphic fabric, is a critical aspect of research and a field where new advances achieved due to the rapid development and availability of advanced technologies such as image analysis and remote sensing techniques of microlevel identification of various phases [30, 31, 32]. The combination of remote sensing techniques, Geographic Information Systems (GIS), modeling with microanalytical techniques successfully applied within classical igneous petrology, but in recent times, metamorphic geology also follows these trends to benefit from the seemingly distant disciplines.

Metamorphic geology is also closely linked to structural geology (Figure 1). Most of the metamorphic textures are the most complex 3D textural and structural assemblages from landscape to microscales (Figures 2 and 3). While traditional mapping is still the key for recording and interpreting such complex textures in micro to landscape scales, the fusion of various high resolution digital elevation modeling, including LiDAR and structure for motions (SfM) photogrammetry tools allowing us to reach mm-scale resolution of 3D models capable to capture fine details earlier, was nearly impossible [33, 34, 35]. While the combination of metamorphic and structural geology is still in its infantry by application, the advanced technologies, the direction is clear. Other than visible light sensors, multispectral analysis or laser scanning are just among few main tracks, and high-resolution structural analysis is possible. Combining the high-resolution spatial data from micro to landscape scale with the advanced chemical tools through machine learning and AI will clearly provide a revolution within metamorphic geology to understand the evolution of various lithosphere segments and identify key patterns.

Figure 1.

The South Island of New Zealand 1 to 1 million scale geological map overlaid on the 8 m resolution digital terrain model (LINZ) demonstrating well the connection between the structural elements and morphology of the region. The metamorphic grade increases toward the main active fault (Alpine Fault) within the Permo-Triassic Eastern Province’s Rakaia Terrain. Higher grade greenschist zones marked by purple grades are already part of another terrain, the Triassic Eastern Provinces’ Caples Terrain. For clarity, large lakes are marked with black fields.

Figure 2.

Higher grade greenschist along the Alpine Fault in the South Island of New Zealand.

Figure 3.

Strong deformation along the Alpine Fault in the outcrop scale offering perfect opportunity to use high-resolution advanced technologies including applied remote sensing of mapping the link between structural patterns and metamorphic zones.

Metamorphic geology is also just in the first steps to be more known within geoheritage and geodiversity research [36, 37, 38]. As our planet facing with global and planetary changes, metamorphism is one fundamental aspect of geology that produced numerous geoheritages where dedicated metamorphic geology heritage sites should be systematically explored. We will likely experience an explosion of the number of such research in the future. Overall, metamorphic geology has experienced dramatic advances in recent times, and the recognition of these steps is important in recurrent scientific report such as this InTech book.