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In the present study, Paleozoic Variscan orogenesis was a model of the oroclinal flexion accompanied by extensive magmatism, which could be divided into the following two types: post-tectonic and syn-tectonic tonalite granite, and leuco-granite which were controlled by the tectonic characteristics of the intrusions. It was observed that a very high majority of the samples had displayed discontinuities in their structures, that were later utilized to define the granitoid morphology and development characteristics of the rock during the intrusion phases. Furthermore, it was determined that the tectonics associated with the Alpine orogeny results in the new generation of faults and fractures during the Paleogene Period had produced the development of the Sierras. Due to different weathering processes, the depressions which had resulted in the present granitoid reliefs were found to be exclusively related to the structural development processes during the geological history (either tectonic or magmatic) of the granite, and not as normally interpreted.
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Journal of Earth Science, Vol. xx, No. x, p. xxx–xxx, online 2020 ISSN 1674-487X
Printed in China
https://doi.org/10.1007/s12583-019-1268-z
Vidal Romaní, J. R., Song, Z. J., Liu, H. M., et al., 2020. Orogenic Movements during the Paleozoic Period: Development of the Granitoid
Formations in the Northwestern Region of Spain’s Iberian Peninsula. xx(x): xxx–xxx. https://doi.org/10.1007/s12583-019-1268-z.
http://en.earth-science.net
Orogenic Movements during the Paleozoic Period:
Development of the Granitoid Formations in the Northwestern
Region of Spain’s Iberian Peninsula
Juan Ramon Vidal Romaní *1, 2, Zhaojun Song *1, 3, Huimin Liu1, Yifang Sun1, Haonan Li1
1. College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao 266590, China
2. University Institute of Geology, University of Corunna, Galicia 15071, Spain
3. Laboratory for Marine Geology, Qingdao National Laboratory for Marine Science and Technology, Qingdao 266061, China
Juan Ramon Vidal Romaní: https://orcid.org/0000-0001-7158-4945; Zhaojun Song: https://orcid.org/0000-0001-6516-2818
ABSTRACT: In the present study, Paleozoic Variscan orogenesis was a model of the oroclinal flexion ac-
companied by extensive magmatism, which could be divided into the following two types: post-tectonic
and syn-tectonic tonalite granite, and leuco-granite which were controlled by the tectonic characteristics of
the intrusions. It was observed that a very high majority of the samples had displayed discontinuities in
their structures, that were later utilized to define the granitoid morphology and development characteris-
tics of the rock during the intrusion phases. Furthermore, it was determined that the tectonics associated
with the Alpine orogeny results in the new generation of faults and fractures during the Paleogene Period
had produced the development of the Sierras. Due to different weathering processes, the depressions
which had resulted in the present granitoid reliefs were found to be exclusively related to the structural
development processes during the geological history (either tectonic or magmatic) of the granite, and not
as normally interpreted.
KEY WORDS: granitoid, geomorphology, endogenous forms, exogenous forms, intrusive structures.
0 INTRODUCTION
Oroclines are considered to be among the largest geological
structures on Earth, with formations having developed from the
Archean Period to the present time. The Cantabrian Orocline
located in northern Spain is one of the first orogens reported in
researches, and was generally referred to as the “Asturian Knee”
by Eduard Suess (Suess, 1883) in the late nineteenth century. The
structure has been attributed to Early Carboniferous collisions
between Laurussia and Gondwana during the Pangea amalga-
mation (Fig. 1), and is defined by a significant bend in the
northern Iberian Peninsula.
The Cantabrian Orocline tends to weave its way through
western Europe, and is located at the apex of the Ibero-
Armorican arc. The Carboniferous Variscan orogeny is accom-
panied by extensive magmatism in response to active buckling.
It is known that the synorogenic Variscan granitoid magmatism
was active from 345 to 315 Ma and from subsequent post-
orogenic magmatism emplaced from 310 to 285 Ma (Weil et al.,
2013; Auréjac et al., 2004). The post-orogenic magmatic in-
cluded significant foreland magmatism in the core of the
*Corresponding author: juan.vidal.romani@udc.es;
songzhaojun76@163.com
© China University of Geosciences (Wuhan) and Springer-Verlag
GmbH Germany, Part of Springer Nature 2020
Manuscript received September 25, 2019.
Manuscript accepted November 6, 2019.
Cantabrian Orocline (Fig. 2).
The present relief of Galicia has developed on the afore-
mentioned surface. Its age has been determined to be approxi-
mately 200 Ma (Mesozoic Period) (Vidal Romaní et al., 2014,
1998), and presents significantly more modern structures than the
rock masses (predominantly granite, slate, and schist) on which it
developed, which date back to the Paleozoic Period (Liu et al.,
2018; Meng et al., 2018; Li et al., 2017; Wang et al., 2016; Han et
al., 2014). During the Mesozoic Period the mega-continent of
Pangea was divided into many portions, one of which corre-
sponded to the basement rock formations underlying the region
of Galicia (Fuenlabrada Pérez, 2018). Since the Galician territo-
ries were exposed on the surface, they had been mainly affected
by fluvial erosive processes (Vidal Romaní et al., 2014, 1998),
and less intensely affected by marine and glacial processes.
During the Mesozoic Period, the Galician rivers began to flow
towards the present-day Atlantic Ocean coastal regions, digging
deeply into the valleys which had been submerged by the sea
during their final stretch in the Cainozoic marine transgressions,
becoming the area currently known as “Rias” (Vidal Romaní et
al., 1998). The most important milestone in the landscape evolu-
tion of Galicia dates back to the Paleogene Period (65–34 Ma),
when the Iberian Peninsula (a small plate between the Eurasian
Plate and the African Plate) was affected by tectonic activities
which gave rise to the different current Galician mountain ranges,
largely sustained by granitoids (Vidal Romaní et al., 2014).
Among the aforementioned mountain ranges, over 1 000 m had
been developed during the glacial processes which occurred
Juan Ramon Vidal Romaní, Zhaojun Song, Huimin Liu, Yifang Sun and Haonan Li
2
during the Quaternary Period (2.58–15 ka) in the far areas of
western Europe (Vidal Romaní et al., 2014).
1 LANDFORMS OF GRANITE TERRAINS
For more than two centuries, the formation and landscape
development of granite and granitoids have attracted extensive
attention from geologists, geographers, and geomorphologists
(Song et al., 2019). In addition, almost without exception, each of
the previous studies had assumed that granite rocky landscapes
had been formed by weathering and rock erosion, thereby high-
lighting the importance of climate in the end results. This had
become the case for the monographies of granite formations,
with emphasis placed on the influences of climate on their for-
mations (Büdel, 1977; Godard, 1977; Ollier, 1969; Wilhelmy,
1958). This was then highlighted by delightful miniatures of
block-diagrams, representing different types of granite climatic
landscapes. Then, approximately 37 years ago, Twidale (1982)
published the first specialized treatise on granite morphology,
which was subsequently updated, revised, and improved (Migon,
2006; Twidale et al., 2005; Vidal Romaní et al., 1998). As a result,
one might think that after all the significant advances mentioned
above, the interpretations of the genesis of the granitoid forma-
tions have already been resolved. However, this is not the case, as
there are still many unanswered questions regarding the
Figure 1. Collision between Gondwana and Laurussia in the Late Devonian between 380 and 370 Ma. The approximate situation of the Iberian Peninsula
(Spain and Portugal) was located in the rectangle marked. Modified from Fuenlabrada Pérez (2018).
Orogenic Movements during the Paleozoic Period: Development of the Granitoid Formations
3
Figure 2. Geological map of Galicia with the location of the main granite bodies (modified from Weil et al., 2013).
origins of tafone, polygonal cracking, pseudo-bedding, and so
on. Therefore, any observer contemplating granitoid rocky
landscapes tended to appreciate specific common features
which potentially insured the accuracy of specific granitoid
geomorphology cases. Furthermore, what is more surprising,
Twidale et al. (2005) noted that there were azonal characteris-
tics in the formations. That is to say, associations of forms
which were common to all the granitoid rocky landscapes were
observed, which were found to be independent of the climate or
external geodynamic agents which exist today, or of those
which acted in the past (Twidale et al., 2005).
Our climate has varied greatly throughout the geological
history of the Earth, either for astronomical reasons or because
of the migration of lithospheric plates (Nonn, 1966). Therefore,
an inselberg or a bornhardt, according to classical geomor-
phology (Bremer and Jennings, 1978) which originally formed
in desert zones and were now under tropical, cold, temperate,
or humid climatic conditions could be understood as cases of
inheritance. For example, this would include climatic forma-
tions which do not correspond the present climate conditions in
their current locations.
The almost unanimous assertion is that all of the formations
which had developed on granite or granitoid rock masses have
exogenous origins, having been formed by the actions of exoge-
nous processes, and were considered to be practically eternal if
permanent contact with water had been avoided.
2 ENDOGENOUS ROUTE
However, it should be noted that there is another option to
explain the morphological similarities of granite and granitoid
rocky landscapes. It has previously been confirmed that mag-
matic rock tends to form in the interior of the lithosphere from
which it is mobilized and then moves toward the superficial
levels. These rock formations can reach the Earth’s surface as
extrusive or volcanic rock, thereby developing the very specific
and unequivocal geomorphological characteristics widely
described in previously research results (Ahmed et al., 2018;
He et al., 2018; Wang et al., 2018). In other cases, the mag-
matic rocks which become consolidated in the interior layers of
the Earth tend to give rise to different types of intrusive rock
formations (granite) according to their mineralogy, textures,
and structures. In addition, near the end of the intrusive stage
(Vidal Romaní, 2008; Auréjac et al., 2004), a very specific
structural fabric (discontinuities) will be generated. Once the
rock has been exposed on the Earth’s surface, the structural
fabric (discontinuities) will define the rocky reliefs with a
specific morphology for the granitoid reliefs. The novelty of
this interpretation lies in the fact that the structural fabric of the
plutonic rock does not develop on the surface of the Earth or
within the deeper layers, but during the formations’ ascent
Juan Ramon Vidal Romaní, Zhaojun Song, Huimin Liu, Yifang Sun and Haonan Li
4
toward the surface of the Earth and always within the litho-
sphere, as can be observed in different areas of China (Meng et
al., 2019; Han et al., 2014; Zhu et al., 2014). In any of the cases
near the ends of the aforementioned ascension processes, ero-
sion actions are required. The erosion actions normally respect
the original contours of the intrusive magmatic bodies and
eliminate the host rock, essentially leaving the granitoid ex-
posed on the surface.
3 GENERATION OF THE PLUTONIC ROCK MASS
Granitoid rocks, such as granite sensu stricto and its close
petrological relatives (Streckeisen, 1967), are material expelled
from the interior layers of the Earth. Once the continents were
formed, the plate tectonic fragments had dispersed the granite
outcrops throughout the terrestrial surface. Such dispersing actions
are known to have occurred after the disintegration of the last
great supercontinent (Pangea) and continued until the present time
(Ganne and Feng, 2018). Plutons are classified partially by their
sizes and shapes. However, they are mainly categorized according
to whether they are predominantly discordant (post-kinematics) or
concordant (syn-kinematics) in respect to the structural character-
istics of the host rock. The most suitable geodynamic environ-
ments for finding extrusive and intrusive magmatic rock forma-
tions are the mid-oceanic ridges, oceanic-continental island arches,
oceanic-oceanic island arches, continental plate edges, hot spots
and magmatism due to meteorite collision (Ulrich et al., 2018).
However, over time, the accretion processes formed by the colli-
sion between plates had resulted in the increasing integration of
the plutonic bodies toward the interior of the continents. Such was
the case of the north western section of the Iberian Peninsula.
3.1 Intrusion Occurrences of the Magmatic Rock
Recently, it has been explicitly recognized that the move-
ments of granitoid rock masses on their way to the Earth’s sur-
face are not in a fluid or molten state (Liu et al., 2019), but in
crystallized solid (mush) formations with variable proportions of
volatile (between 5% and 20%) content (Vidal Romaní et al.,
2018; Cashman et al., 2017; Parmigiani et al., 2016). The volatile
content results in solid poly-mineral deposits, and already totally
crystallized masses, with some peculiar rheological properties
which allow movements similar to that of very viscous fluid
(Vidal Romaní et al., 2018; Cashman et al., 2017; Parmigiani et
al., 2016).
In the aforementioned processes, the magma migrates fol-
lowing the fissures or channels (chimneys) formed during the
movements of the lithospheric plates. The materials which circu-
late through the channels are mush rather than molten materials,
with masses practically in solid states although volatile residual
fractions (between 5% and 20%) are conserved. This confers the
moving materials with a great capacity to deform when subjected
to directed effects (Cashman et al., 2017) (Fig. 3). During the
intrusive stage, the mush will circulate in a forced regime
through the predefined fissures or conduits. The first conse-
quences of these movements will be the generation of fluid
structures of endogenous magmatic origin, which then culminate
in the total consolidation of the mush in plutonic rocks (granite).
Petrologists have advanced different models (Cashman et al.,
2017) to explain the movements of the mush on a large scale,
although the process has also been theoretically defined on a
smaller scale in a Hele Shaw cell (Figs. 4a, 4b) (Bensimon et al.,
1986; Liang, 1986a, b). Therefore, it is possible to theoretically
establish how the mush moves during each previous stage in
order to reach the final consolidation stage using the aforemen-
tioned model. The authors are particularly interested in the as-
sumptions presented by Liang (1986a, b) (see also Bensimon et
al., 1986), in which the movements of the “mush” were produced
in the form of cylindrical digitations of approximately circular
cross-sections, which were propagated according to a mode of
telescopic advance.
In previous studies, in-situ cases of the above-mentioned
stage were examined in some rocky outcrops. It was found that
the Northwest Iberian region, where there are known to have
been a large number of intrusions of granitoid bodies during the
two stages of the Variscan orogeny, would undoubtedly provide
an ideal location for the type of observational examinations
mentioned above (Fig. 5). Since it was confirmed that this was
the case in Pontevedra (Spain) (see Fig. 5), it was possible to
observe magnificent examples of this type of intrusive structures.
It was found that the contours of the ducts were marked by en-
veloping surfaces (films) of phenocrystals, which allowed for the
definitions of the dimensions of the chimneys where the mush
ascended, as well as the reconstruction of the directions of the
magma movements in the area. At the present time, the largest
observed structures have diameters of approximately 3 m. In
addition, other cases with similar characteristics have been de-
scribed in northern Portugal (Valpaços, Valverde). However, it is
also possible to observe these types structures which mark the
movements of mush as enormous ellipsoids (Fig. 6) formed by
the total crystallization of granite formations. Previous related
studies have very frequently described movements of mush in the
xenoliths and xenocrystals, as well as the accumulated and en-
clave (Corretgé et al., 1984) structures of granite outcrops. These
magmatic enclaves of discrete dimensions appear as 3-axis ellip-
soids with their major axis oriented in the direction (generally
vertical) of the movement of the magma. These are normally up
to 20 or 30 cm in diameter, although in exceptional cases, they
have been observed to reach up to 2 m in diameter. Therefore, it
is a possibility that these ellipsoids were formed by the crystalli-
zation and nucleation of the mush (Cashman et al., 2017) around
previous solid nuclei, either xenoliths or xenocrystals, or were an
aggregation of the mineral enclaves. In any case, the chimneys or
ducts, as well as the enclaves themselves, had been formed dur-
ing the magmatic stages (Twidale et al., 1994), and had no deci-
sive influences on the morphology of the final granitoid bodies.
However, it remains unclear what occurs when erosion ac-
tions eventually place these granite formations on the Earth’s
surface. Generally speaking, it has been found that they present
two types of morphology (Twidale, 1971; Badgley, 1965): either
discordant (post-tectonic) domes or concordant (syn-tectonic)
(laccoliths) granitoid bodies (Cashman et al., 2017).
3.2 Concordant Syn-Tectonic Magmatic Bodies
In the cases of syn-tectonic magmatic bodies, their structural
characteristics are only observable after erosion has eliminated
the host rock, leaving the granitoid exposed on the surface. From
the aspect of morphology, they tend to display reliefs formed by
Orogenic Movements during the Paleozoic and the Development of Granitoid Forms
5
Figure 3. Two modes of granitoid body ascending through the lithosphere in its two modalities: concordant (laccolith) and discordant (dome or neck) (modified
from Cashman et al., 2017).
Figure 4. Different modes of mush movements through the upper lithosphere (Liang, 1986a). (a) Single Hele Shaw cell; (b) branched Hele Shaw.
Figure 5. Field photographs showing the cross sections of a single Hele Shaw of granitoid (Pontevedra, Galicia).
Juan Ramon Vidal Romaní, Zhaojun Song, Huimin Liu, Yifang Sun and Haonan Li
6
Figure 6. Flow of granitoid mush in concordant granitoid from Santa Valha
(Portugal) showed by a granitoid ellipsoid with concentric envelopes.
Elongation axis parallel to the direction of movement.
Figure 7. Surface morphology of a granitoid body concordant with its
largest dimension parallel to the Earthʼs surface from Valverde, Portugal.
Figure 8. Granitoid dome as an example of a discordant body. O Pindo,
Galicia, Spain.
lowered bodies, with their greater flat dimensions parallel to the
surface of the land (Sánchez Cela, 2004). In regard to the lacco-
liths, the final intrusions have taken place in domains close to the
Earth’s surface. As a result, the effects of the low confining
pressure (Cashman et al., 2017) favours the lateral extension of
the granite mass that adopts a very characteristic tabular mor-
phology. The dominant internal structure in this type of granitoid
bodies is the so-called sheet structure, formed by the injection of
plutonic rock (mush) into the previous terrain in superimposed
sheets. The injection always implies a friction of the intrusive
with the host terrain or with itself in each layer. In transversal
section these concordant bodies are shown as large parallel
sub-horizontal sheets of variable thickness (between half a meter
or less to a few meters), which indicates that it was produced by a
successive injection. The contact between each sheet can be net
or gradual as indicated by a shear zone with a variable thickness
from a few centimeters to several meters in some cases. There the
rock is finely divided into sheets or planar scales formed during a
tectonic shearing process. They have a “morphological” similar-
ity to a porphyroblastic gneissic structure although here the
dimensions are much larger in metric order (Fig. 7). Then, when
the later erosion made the sheared granite disappear, the intact
granite “eyes” became loose. On occasion, remains of the foli-
ated rock which originally enveloped the granite have been
observed. Some studies (Vidal Romaní, 2008) have also associ-
ated these zones with other typical shear structures, such as
polygonal cracking, which are only obviously conserved on the
surfaces of the granite blocks in some cases, or on different faces
of the blocks. In such cases, polygonal cracking is associated
with the development of tafoni, not only at the base of the granite
blocks, but in any or even all of the block faces. This is very
characteristic of the association of structures with the following
order of generation: shearing, polygonal cracking, and the de-
velopment of tafoni. This type of desquamation structure or sheet
structure formed by scales (leaves) of intact rock and of great
radius defines a very characteristic landscape of domes of very
low relief (almost flat) which interfere laterally between them by
the juxtaposition/imbrication of the sheet structure.
3.3 Discordant Magmatic Bodies
However, when the intrusion of the mush occurs at a greater
depth under a higher confinement pressure conditions, the grani-
toid bodies will definitely consolidate at greater depths. In such
cases, the intrusions define bodies of smaller dimensions, with
external contours of the ellipsoidal or cylindrical sections (cir-
cumscribed massifs; domes) (Fig. 8) (Cashman et al., 2017). The
elongation axis will generally be vertical and may sometimes
intersect through some previous laccoliths. Although the cylin-
drical domed bodies are not as frequent as the laccoliths, they are
the best known, perhaps due to the dramatic aspects they present
when erosion exposes them on the surface. Therefore, the fol-
lowing tend to be highlighted in the surrounding reliefs of the
totemic shapes in granite landscapes: domes, inselbergs, or
bornhardts.
3.4 Bornhardts, Domes and Other Formations
Without a doubt, during the formation development of
granitoid massifs, the most relevant structural characteristic of
the morphologically includes the exfoliation (sheet structures)
since it is the one which defines the bornhardt dome reliefs,
which are considered to be the most notorious form (although not
the only exclusive form of granitoid reliefs) (Fig. 9) (Twidale et
al., 2005). The bornhardts (Rey et al., 2011; Twidale et al., 2005,
1994) are the last remnants which have survived the long-
distance scarp retreat processes. This is due to their massive
structures within the innermost areas of the granite bodies, which
have not been affected by the discontinuities associated with
contacts with the host rock. Both interpretations are partially
correct since when the erosion has eliminated the external sec-
tions, or essentially the ones most affected by the discontinuity
Orogenic Movements during the Paleozoic and the Development of Granitoid Forms
7
systems, while only the internal and more massive parts of the
pluton will remain. These inner sections have resisted weathering
effects due to the absence of structures (discontinuities). However,
as noted in all of the examined cases, the sheet structures in the
bornhardts developed when the rock had become completely
crystallized, and could potentially even affect the host rock.
Some previous researchers (Tahiri et al., 2007) have referred to
the deformative structures developed between the pluton and the
host rock contacts as “the strain aureole”, considering them even
as “magmatic structures developed in solid states” (Petford, 2003;
Diot et al., 1987). Therefore, it has been implicitly suggested that
the rock was already consolidated when the discontinuities de-
veloped (Zulauf G et al., 2011; Zulauf J et al., 2011). This fact
has also been corroborated by other results (Vidal Romaní, 2008;
Twidale et al., 2005, 1994; Vidal Romaní and Twidale, 1999), in
which it was considered that the sheet structures were not the
only type of discontinuity, but one more of those which had
developed in gradual series of increasing magnitudes due to shear
deformations in the contacted granitoid host rock. Most impor-
tantly, their development occurred when the rock had become
fully crystallized. The shear deformations were limited to the
host rock-granitoid contact positions, and gave rise to different
types of associated discontinuities, such as lamination, granitoid
foliation, pseudo bedding, and in the most extreme cases, a type
of deformation referred to as boudinage and polygonal cracking
(Zulauf G et al., 2011; Zulauf J et al., 2011; Vidal Romaní, 2008;
Vidal Romaní and Twidale, 1999) (Fig. 10). Therefore, all these
types of discontinuities were confirmed to not be related to the
unloading of the massifs, but are now considered to be shear
structures confined to the immediate contact zones between
plutonic and host rock. Furthermore, their development is now
known to have taken place inside the lithosphere, as evidenced
by the fact that in many cases the contacts of the aforementioned
discontinuities have been injected by pegmatite, quartz, and
leuko-granite bodies, and so on (Tahiri et al., 2007; Twidale,
1982). In addition, the internal domes have characteristic internal
structures which are defined by two main types of discontinuities.
The first type is developed in the internal zones characterized by
having the furthest the contacts with the host rock. In those loca-
tions, fluid structures are commonly found, although the rock is
totally crystallized. In such cases, the movement implies that the
flow is made in a solid state (Rodrigues Waldherr et al., 2018;
Vidal Romaní et al., 2018).
However, it is more common that the structure developed
in contact with the walls of the ducts (sheet structures) (Vidal
Romaní, 2004; Vidal Romaní and Twidale, 1999), although
variable dips are usually observed in those cases. Some re-
searchers have interpreted the aforementioned as structures of
relaxation or erosive unloading. However, others (Vidal Ro-
maní and Yepes Temiño, 2004; Vidal Romaní and Twidale,
1999) have decisively associated them with shearing effects
located in the contact zones of the granitoid and host rock
which had developed during the intrusive stages of the grani-
toid. According to the aforementioned research interpretations
(Vidal Romaní and Yepes Temiño, 2004; Vidal Romaní and
Twidale, 1999), zoning of the sheet structures exist (Figs. 7 and
8) which leads to its gradual disappearance (fading) from its
best development points (in contact with the host rock) to its
Figure 9. Sheet structure in granite from Ézaro, O Pindo, Galicia, Spain.
Figure 10. Boudinage-polygonal cracking on the surface wall of a granitoid
dome as proof of the intrusive origin of the dome from Arouca, North Portugal.
total disappearance inside the granitoid domes.
3.5 Fracturing or Discontinuity Systems
The most decisive properties of granitoid rock masses in-
clude their impermeability, low porosity, low solubility, and
excellent resistance to weathering when dry (Vidal Romaní et al.,
2015). These features explain the importance of the discontinuity
systems in this type of rock since when open those systems direct
movement toward the interiors of the rock massifs resulting in
alterations in the rock. As a consequence of these movements, the
distributions of the discontinuities or the fracturing systems tend
to control the final morphology of the rocky landscapes devel-
oped on granite deposits. However, until recently, the origins of
these discontinuities have been misinterpreted (Burton Johnson et
al., 2019) due to the lack of understanding, whether implicitly or
explicitly, of their formation processes related to the subsequent
intrusion-consolidation of the rock. As mentioned above, in
magmatic (plutonic) rock, discontinuities are formed on the
already consolidated rock formations. However, these are gener-
ally not evident on the Earth’s surface, but as granite intrusions
formed through lithosphere progresses. This explains why the
morphological patterns which are defined by the discontinuities
of granitoid bodies consistently result in very similar formations
due to the intrusions always occurring in the same way. Perhaps
that is why the representations of the discontinuity systems which
define the plutonic rock bodies are almost always presented (see
Brook, 1978) with the same generic scheme. For example, grani-
toid domes of radial and anticline structures or systems of conju-
gated discontinuities formed by two or more orthogonal families,
Juan Ramon Vidal Romaní, Zhaojun Song, Huimin Liu, Yifang Sun and Haonan Li
8
almost always perpendicular to the Earth’s surface and which
(Cloos, 1931, 1923) are invariably related to the flow tray of the
batholiths (although this almost never happens in all real cases).
The most obvious types of discontinuities are sheet structures,
which have been identified as structures of relaxation, decom-
pression, or erosive discharges associated with the dome shapes
observed in the contact zones of granite/granitoid deposits and
their host rocks. However for some previous related studies
(Vidal Romaní, 2008, 2004; Vidal Romaní and Twidale, 1999)
have theorized that the formations of sheet structures do not
correspond to the effects of unloading, but to the effects of com-
pressive actions in environments dominated by shear stress, as
confirmed by the minor structures associated with the contacts
between the structural units (leaves) of the sheet structure. These
include pseudo bedding, boudinage, polygonal cracking, and
even with the formation of tafone (Fig. 10) (Vidal Romaní, 2008,
2004). However, regardless of these factors, there still are many
researchers who point out the similarity of the granitoid-salt
diapirs in order to explain the intrusions of discordant plutonic
rock bodies based on the similarities in the structural and mor-
phology characteristics (Vendeville et al., 1992; Talbot and Jack-
son, 1987) in granitoid domes. In the aforementioned theories,
the related studies have ignored that, although the intrusive proc-
esses seem mechanically similar, the characteristics of the mate-
rial and geodynamic environments (lithostatic pressure, tem-
perature, density) involved in both cases tend to be very different.
4 CONCLUSIONS
In regard to granitoid bodies, either concordant or discor-
dant, it has been found that once they arrive at the Earth’s
surface, they are characterized by a very specific morphology
which takes advantage of the preferable systems of discontinui-
ties affecting the rock. These discontinuities are defined during
the last intrusive stages of the granitoid and are therefore of
endogenous origin. During these endogenous stages, even
though total crystallization has occurred, the rock masses have
been observed to have very specific rheological behaviors in
the proximity of the contact zones of the magmatic bodies and
the host rocks. These contacts propitiate deformations by
ductile-fragile shearing actions which generate very specific
planar structural fabric (Vidal Romaní, 2008; Glazner et al.,
2004). For example, having been previously defined for sedi-
mentary or metasedimentary rock masses (Zulauf G et al., 2011;
Zulauf J et al., 2011; Ramsay and Huber, 1987) and being un-
equivocally attributed to deformative processes developed
during the folding of the sedimentary masses, these actions are
known to be associated with stratification, fracture planes,
faults, and so on. Until recently plutonic rock masses which
lack stratification planes have been interpreted as sheet struc-
tures (Gutiérrez, 2005) of exogenous origin caused by erosive
unloading processes. However, the erosion processes are now
known to have not eliminated the host rocks (sedimentary or
metamorphic) which were less resistant to erosion until mil-
lions of years after the granitoid had been intruded and de-
formed. Therefore, it can be concluded that the granitoid rocky
landscapes were not evident until the rock became exposed on
the Earth’s surface (Gutiérrez, 2005; Twidale et al., 2005, 1994).
In addition, this essentially explains why the endogenous fea-
tures of granitoid rocky landscapes have been misinterpreted as
exogenous formations caused by erosion.
Once the granitoid rock masses reach the surface layers,
the meteoric agents gave rise to the only exogenous formations
developed on the surface. The most conspicuous cases of the
aforementioned are surfaces polished by marine, fluvial, glacial,
or wind erosion, as well as the generation of pot-holes of fluvial,
marine, or subglacial origin), and grooves and rills caused by
the erosion or dissolution of the rock by water.
ACKNOWLEDGMENTS
This work was financially supported by the National Natural
Science Foundation of China (NSFC) (Nos. 41472155,
41876037), the Laboratory for Marine Geology, Qingdao Na-
tional Laboratory for Marine Science and Technology (No.
MGQNLM201902), the Scientific and Technological Innovation
Project from the China Ocean Mineral Resources R & D Asso-
ciation (No. DY135-N2-1-04). The final publication is available
at Springer via https://doi.org/10.1007/s12583-019-1268-z.
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Large meteorite impact structures on the terrestrial bodies of the Solar System contain pronounced topographic rings, which emerged from uplifted target (crustal) rocks within minutes of impact. To flow rapidly over large distances, these target rocks must have weakened drastically, but they subsequently regained sufficient strength to build and sustain topographic rings. The mechanisms of rock deformation that accomplish such extreme change in mechanical behaviour during cratering are largely unknown and have been debated for decades. Recent drilling of the approximately 200-km-diameter Chicxulub impact structure in Mexico has produced a record of brittle and viscous deformation within its peak-ring rocks. Here we show how catastrophic rock weakening upon impact is followed by an increase in rock strength that culminated in the formation of the peak ring during cratering. The observations point to quasi-continuous rock flow and hence acoustic fluidization as the dominant physical process controlling initial cratering, followed by increasingly localized faulting.
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Paleoproterozoic granulite facies rocks are widely distributed in the North China Craton (NCC). The Huai'an terrane, located within the northern segment of the Trans-North China Orogen (TNCO), a major Paleoproterozoic collisional belt in the central NCC expose mafic and pelitic granulites as well as TTG (tonalite-trondhjemite-granodiorite) gneisses. Here we investigate the pelitic granulites from this complex and identify four distinct mineral assemblages corresponding to different metamorphic stages. The prograde metamorphism (M1) is recorded by relict biotite and the compositional profile of X ca (grt) isopleths. The P max (M2) is distinguished by the X ca (grt) isopleths, which corresponds to the kyanite stable area with an inclusion mineral assemblage of Grt-c–(Ky)-Qz-Rt-Kfs-liq suggesting that the pressures were higher than 12 kbar with a temperature below 900 °C. However, kyanite is absent in thin sections suggesting its consumption during later stages. The T max metamorphism (M3) is characterized by the assemblage: Grt-m-Qz-Pl-Rt-Kfs-Sil-liq in the garnet mantle and also reflected in the compositional profile. Two-feldspar geothermometry yields a P-T range of 940 °C–950 °C and 9.5–10.5 kbar, indicating ultra-high temperature (UHT) metamorphic overprinting. The subsequent retrograde metamorphic stage (M4) is characterized by Grt-r-Bt-Sil-Kfs-Pl-Qz ± Rt ± Ilm with symplectites of Bt-Sil-Qz in the garnet rim suggesting garnet breakdown with P-T conditions estimated as 770 °C–840 °C and 6.5–8 kbar. The pelitic granulites show a clockwise path, with P-T estimates higher than those in estimated in previous studies using conventional techniques. LA-ICP-MS U–Pb analysis of metamorphic zircon grains yield two groups of ages at 1972.9 ± 8.1 Ma and 1873.3 ± 9.9 Ma. We suggest that the protoliths of the Manjinggou HP-UHT granulites were deep subducted where they experienced HP metamorphism associated with the collision of the Ordos and Yinshan blocks at ca. 1.97 Ga. Subsequently, the UHT metamorphic overprint occurred during the assembly of the unified Western and Eastern Blocks of the NCC along the TNCO at ca. 1.87 Ga.
Article
In this study, a SEM method was used to analyze the surface morphology of the quartz sand granitic tafoni at Laoshan, for the purpose of exploring the weathering process of this tafoni. Present study showed that granitic tafoni at Laoshan, the quartz sand roundness was dominated by angular and sub-angular morphologies. Massive Hydrodynamic features had been developed on the quartz sand surfaces, as well as wind and chemistry forms, which were more developed. It was determined that granitic tafoni at Laoshan, the quartz sand had suffered long-term rainy and windy mechanical erosion, as well as chemical dissolution from residual pit water. These findings differed from the earlier views that the tafone was formed by the glacial melt water. © 2019, National Institute of Science Communication and Information Resources (NISCAIR). All rights reserved.
Article
The Early Cretaceous Houyaoyu granite porphyries are located in the south margin of the North China Craton. Field observations, petrography, geochronology, major and trace elemental and Sr-Nd isotopic compositions are reported to elucidate the genesis of the Houyaoyu granite porphyries. SIMS zircon U-Pb analyses for the Houyaoyu granite porphyries yield two concordant ages of 133.2±2.3 (2σ) and 131±1.1 (2σ) Ma, respectively. Major and trace elemental compositions indicate that these porphyries are high-K I-type granites with high contents of SiO2, K2O, Rb, U, Pb, low Nb, Ta, Ti, and P. Initial ⁸⁷Sr/⁸⁶Sr ratios range from 0.708 3 to 0.709 7, and εNd(t) values range from -9.13 to -12.3, with corresponding two-stage depleted-mantle Nd model ages (T2DM) varying from 1.57 to 1.91 Ga. This suggests that the Houyaoyu granite porphyries were predominantly derived from ancient lower continental crust, with minor involvement of mantle-derived components. On the basis of the tectonic evolution of the Qinling Orogen and geochemical characteristics of the Houyaoyu granite porphyries, it is proposed that they were formed in an extensional tectonic setting related to lithospheric destruction of the North China Craton, and produced Mo and Pb-Zn mineralization in East Qinling Orogen.
Article
Mesozoic magmatism is widespread in the eastern South China Block and has a close genetic relationship with intensive polymetallic mineralization. However, proper tectonic driver remains elusive to reconcile the broad intracontinental magmatic province. This study presents integrated zircon U–Pb dating, Hf isotopes and whole-rock geochemistry of the Xiwan dioritic porphyry in the NE Jiangxi ophiolitic mélange. Zircon U–Pb dating by SIMS and LA-ICP-MS methods yielded an emplacement age of ~160 Ma for the Xiwan diorite, confirming its inclusion into the Mesozoic magmatic province in SE China, instead of a component of the Neoproterozoic ophiolitic mélange genetically. The dioritic rocks have low SiO2 (58.08wt%–59.15wt%), and high Na2O (5.00wt%–5.21wt%) and MgO (4.60wt%–5.24wt%) contents with low TFeO/MgO ratios (1.02–1.09). They show an adakitic geochemical affinity but exhibit relatively low Sr/Y ratios (24.8–31.1) and high Y contents (14.6–18.3 ppm) compared to the Dexing adakitic porphyries. In addition, the Xiwan diorites have moderately evolved zircon Hf isotopic compositions (ɛHf(t)=−6.1–−0.1; TDM2=1597–1219 Ma). These elemental and isotopic signatures suggest that the Xiwan diorite formed through partial melting of a remnant arc lower crust (i.e., early Neoproterozoic mafic arcrelated rocks) in response to the underplating of coeval mafic magmas. In conjunction with the temporal-spatial distribution and complex geochemical characteristics of the Mesozoic magmatism, our case study attests to the feasibility of a flat-slab subduction model in developing the broad intracontinental magmatic province in SE China. The flat-slab delamination tends to trigger an asthenospheric upwelling and thus results in extensive partial melting of the overlying lithospheric mantle and lower crustal materials in an extensional setting during the Mesozoic.