# Challenges in isolating primary remanent magnetization from Tethyan carbonate rocks on the Tibetan Plateau: Insight from remagnetized Upper Triassic limestones in the eastern Qiangtang block

Wentao Huang, Michael Jackson, Mark J. Dekkers, Yang Zhang, Bo Zhang, Zhaojie Guo, Guillaume Dupont-Nivet

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## To cite this version:

Wentao Huang, Michael Jackson, Mark J. Dekkers, Yang Zhang, Bo Zhang, et al.. Challenges in isolating primary remanent magnetization from Tethyan carbonate rocks on the Tibetan Plateau: Insight from remagnetized Upper Triassic limestones in the eastern Qiangtang block. Earth and Planetary Science Letters, 2019, 523, pp.115695. 10.1016/j.epsl.2019.06.035. insu-02270563

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Submitted on 11 Aug 2025

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Highlights

* Triassic limestones in eastern Tibet record three episodes of remagnetization.

* The oldest is carried by authigenic magnetite after pyrite oxidization.

* The second resides in authigenic hematite after further pyrite oxidization.

* The latest is viscous overprint in magnetite and chemical remanence in pyrrhotite.

* Rock magnetic and petrographic tests are key to identify carbonate remagnetization.
Challenges in isolating primary remanent magnetization from Tethyan carbonate rocks on the Tibetan Plateau: insight from remagnetized Upper Triassic limestones in the eastern Qiangtang block

Wentao Huang¹; Michael J. Jackson²; Mark J. Dekkers³; Yang Zhang⁴; Bo Zhang⁴; Zhaojie Guo⁴ & Guillaume Dupont-Nivet⁴,⁵,⁶

¹Department of Earth and Environmental Sciences, University of Rochester, Rochester, New York 14627, USA

²Institute for Rock Magnetism, Department of Earth Sciences, University of Minnesota, Minnesota, USA

³Department of Earth Sciences, Utrecht University, Utrecht, The Netherlands

⁴Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, School of Earth and Space Sciences, Peking University, Beijing, China

⁵University of Rennes1, CNRS INSU UMR 6118, Geosciences, Rennes, France

⁶Institute for Earth and Environmental Science, Universität Potsdam, Potsdam, Germany

**Abstract**

Carbonate rocks, widely used for paleomagnetically quantifying the drift history of the Gondwana-derived continental blocks of the Tibetan Plateau and evolution of the Paleo/Meso/Neo-Tethys Oceans, are prone to pervasive remagnetization. Identifying remagnetization is difficult because it is commonly undetectable through the classic paleomagnetic field tests. Here we apply comprehensive paleomagnetic, rock magnetic, and petrographic studies to upper Triassic limestones in the eastern Qiangtang block. Our results reveal that detrital/biogenic magnetite, which may carry the primary natural remanent magnetization (NRM), is rarely preserved in these rocks. In contrast, authigenic magnetite and hematite pseudomorphs after pyrite, and monoclinic pyrrhotite record three episodes of remagnetization. The earliest remagnetization was induced by oxidation of early diagenetic pyrite to magnetite, probably related to the collision between the northeastern Tibetan Plateau and the Qiangtang block after closure of the Paleo-Tethys Ocean in the Late Triassic. The second remagnetization, residing in hematite and minor goethite, which is the further
subsurface oxidation product of pyrite/magnetite, is possibly related to the development of the localized Cenozoic basins soon after India-Asia collision in the Paleocene. The youngest remagnetization is a combination of thermoviscous and chemical remanent magnetization carried by authigenic magnetite and pyrrhotite, respectively. Our analyses suggest that a high supply of organic carbon during carbonate deposition, prevailing sulfate reducing conditions during early diagenesis, and widespread orogenic fluid migration related to crustal shortening during later diagenesis, have altered the primary remanence of the shallow-water Tethyan carbonate rocks of the Tibetan Plateau. We emphasize that all paleomagnetic results from these rocks must be carefully examined for remagnetization before being used for paleogeographic reconstructions. Future paleomagnetic investigations of the carbonate rocks in orogenic belts should be accompanied by thorough rock magnetic and petrographic studies to determine the origin of the NRM.

# 1. Introduction

The Tibetan Plateau is made of a series of continental blocks that rifted away from Gondwana in the south and subsequently drifted towards and collided with Eurasia in the north from Mesozoic to Cenozoic times (Yin and Harrison, 2000). The process was accompanied by opening and closing of the Pale-, Meso-, and Neo-Tethys oceans, resulting in the formation and subsequent deformation of abundant marine carbonate rocks preserved today in the Himalayas and Tibetan Plateau. From north to south (Figs. 1a, b), these continental blocks include the Qiangtang, which welded to the south of northeastern Tibet along the Jinsha suture after closure of the Paleo-Tethys Ocean in the Late Triassic (e.g., Yin and Harrison, 2000; Song et al., 2015); the Lhasa block, which collided with the Qiangtang block along the Bangong-Nujiang suture zone after closure of the Meso-Tethys Ocean in the Late Jurassic to Early Cretaceous (e.g., Kapp et al., 2007); and the Tibetan Himalaya, the northernmost continental unit of the Indian plate, which collided with the Lhasa block along the Indus-Yarlung suture zone after closure of the Neo-Tethys Ocean in the early Paleogene (e.g., Hu et al., 2015). Large-scale south-north motion of these blocks took place during their transfer from Gondwana to Eurasia, which makes paleomagnetism one of the most important tools to decipher the opening and closing of the Tethys Oceans, the drifting and collision of these continental blocks, and the kinematic
60 evolution and tectonic history of the Himalayas and Tibetan Plateau.

61

62 The widely distributed carbonate rocks on the Himalayas, the Lhasa and Qiangtang blocks,
63 have been one of the main targets for paleomagnetic studies to constrain the paleolatitude drift
64 history of these blocks (circles in Fig. 1b). Paleomagnetic studies on these rocks started in the
65 1980s and 1990s (see summary in van Hinsbergen et al. (2012)), and these pioneering studies
66 have been greatly improved recently by acquiring larger datasets and applying more
67 sophisticated analytical techniques for the Himalayas (e.g., Huang et al., 2015), the Lhasa block
68 (e.g., Bian et al., 2017; Ma et al., 2018), and the Qiangtang block (e.g., Cao et al., 2019).
69 Nevertheless, results reported from these carbonate rocks are disparate, and often contradict
70 the usually more robust paleomagnetic results determined from coeval volcanic and fine-
71 grained detrital rocks. For example, estimated Triassic paleolatitudes of the Tibetan Himalaya
72 based on data from carbonate rocks range from 75.7°S to 24.8°S (reference location: 32.8°N,
73 96.7°E, same for below) (Fig. 1b), and most deviate from the predicted paleolatitudes from the
74 global apparent polar wander path (GAPWaP, Torsvik et al., 2012). Paleomagnetic results from
75 carbonate rocks have been interpreted to indicate that the Lhasa block was located at low
76 southern hemisphere latitudes during Carboniferous-Triassic times, whereas the GAPWaP of
77 Gondwana predicts middle-high southern hemisphere latitude (Fig. 1b). The Late Triassic
78 latitude of the Qiangtang block was constrained to be ~15°N from data from limestones (Zhou
79 et al., 2017), much lower than the robust results of 30-33°N from volcanic rocks (Fig. 1b) (Song
80 et al., 2015).

81

82 Although potential inclination shallowing induced by compaction might contribute to the
83 abnormality in these datasets (Kodama, 2012), widespread remagnetization of carbonate rocks,
84 as recognized in the orogenic belts in North America, South America, and Europe (Elmore et
85 al., 2012; van der Voo and Torsvik, 2012, and references therein), must also be considered.
86 Remagnetization of carbonate rocks has been documented in the Himalayas of the India-Asia
87 convergence zone, which significantly affects constraints on the dimension of Greater India
88 and thus our understanding of the intricacies of the India-Asia collision. In the Himalayan
89 metacarbonate rocks, pervasive remagnetizations residing in pyrrhotite, formed by
desulphidation of pyrite or breakdown of magnetite under reducing conditions, have been identified and postdate the main Cenozoic Himalayan folding (Appel et al., 2012, and references therein). In the Jurassic-Lower Paleogene Himalayan limestones that have not been metamorphosed, we have found that oxidation of early diagenetic pyrite to authigenic magnetite has induced pervasive remagnetization of these rocks (Huang et al., 2015, 2017a,b). This lead us to propose rock magnetic and petrographic criteria for identifying remagnetization in limestones, and these criteria are invaluable when routine field tests cannot be trusted (Huang et al., 2017b). Indeed, although most paleomagnetic studies on these Carboniferous-Jurassic limestones from the Tethyan Himalaya, Lhasa, and Qiangtang blocks have provided positive fold or reversals tests to support a primary origin of their isolated characteristic remanent magnetization, these test do not preclude remagnetization during lithification of flat lying sediments much later than the deposition attending closure of Paleo/Meso/Neo-Tethys Oceans and subsequent continental collisions. Remagnetization processes may have expanded dual geomagnetic polarities, yielding apparent positive reversals tests. The uncertainties in the classic field tests call for determining the origin of the NRM by investigating the characteristics of its carrier(s), which can actually be accomplished by thorough rock magnetic analyses and petrographic observations.

To test whether primary NRMs are preserved, and to identify the mechanisms for potential remagnetization, we present paleomagnetic, rock magnetic, and petrographic results from Upper Triassic limestones in the eastern Qiangtang block of the Tibetan Plateau. We isolated remanent magnetizations, applied high-temperature, room-temperature, and low-temperature rock magnetic experiments, and examined thin sections by scanning electron microscopy (SEM) and energy-dispersive X-ray spectrometry (EDS). Collectively, we evaluated the origin of the remanent magnetizations, interpreted their acquisition mechanisms, and discussed the implications for preservation of primary remanent magnetization in the Tethyan carbonate rocks in the Tibetan Plateau.

## 2. Geologic background and paleomagnetic sampling

The Qiangtang block, one of the major Gondwana-originated units of the Tibetan Plateau, is
bounded by the Jinsha suture to the north and Bangong-Nujiang suture to the south (Fig. 1a).

It is east-west-trending with a maximum width of 400-500 km in the western and middle parts,
but changes into a north-south orientation and narrows to <150 km in the eastern part (Fig. 1a).

The study area is located in the north of the Yushu-Nangqian area in the eastern Qiangtang
block, where the major stratigraphic units are Upper Triassic shallow-water marine carbonates
and minor clastic rocks, deposited on the northern passive margin of the Qiangtang block, and
overlying Paleogene red beds (Fig. 1c, Spurlin et al., 2005). The Upper Triassic strata are the
Jieza Group (T3jz), which contains four units of T3jz¹, T3jz², T3jz³, and T3jz⁴ from bottom to
top. The T3jz¹ consists of sandstone, siltstone, and minor carbonate and volcanic rocks; The
T3jz³ is dominated by mudstone and shale; The T3jz² and T3jz⁴ are composed of carbonate rocks.

Cenozoic strata unconformably overlie the Jieza Group, and contain conglomerate, sandstone,
and minor mudstone and carbonate. Pre-Cenozoic deformation of the Upper Triassic strata is
indicated by the angular unconformity between tightly folded Upper Triassic and mostly flat-
lying Cenozoic strata (Fig. 1c), as along the Ganzi-Yushu fault at the northern margin of the
Shanlaxiu basin, where carbonate deformation was proposed to relate to Late Triassic closure
of the Paleo-Tethys ocean (Spurlin et al., 2005). Cenozoic folding and thrusting of the Upper
Triassic strata are also common, as best observed along the Shanglaxiu thrust farther south, and
can be clearly distinguished from an earlier deformation phase (Fig. 1c).

138

We have focused on limestones from the well-exposed T3jz² along road cut exposures. Standard
paleomagnetic cores with a diameter of 2.5 cm were collected at four sites (14XL19, 14XL20,
15XL23, and 15XL24) using a portable gasoline-powered drill (Fig. 1c). Samples were
oriented with magnetic and sun compasses. In each site, the distance between adjacent cores is
0.5-1 m in stratigraphic level. Specially, there are fourteen, eighteen, and sixteen cores collected
from 14XL19, 15XL23, and 15XL24, respectively. These three sites are non-reddish and far
away from the Cenozoic strata. The other site (14XL20) is reddish and lies just below the
unconformity between the T3jz² and Cenozoic Shanglaxiu basin strata and eighteen cores were
collected.

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3. Paleomagnetism
We isolated remanent magnetizations from these limestones by progressive thermal demagnetization. Successive temperature steps at 130, 170, 210, 250, 290, 330, 370, 410, 450, 480, 510, 540, 570, and 600°C were applied for samples from 14XL19, 15 XL23, and 15XL24, and steps at 130, 170, 210, 250, 290, 330, 370, 410, 450, 490, 530, 570, 610, 640, and 670°C were applied for the reddish samples from 14XL20, until complete demagnetization of the NRM. Remanence was measured using a 2G Enterprises DC SQUID magnetometer (noise level 1-2 x 10⁻¹² Am²) installed in the magnetically shielded room at University of Rennes1 (France). Principal component analysis was used to precisely determine the direction of remanent magnetizations (Kirschvink, 1980). The defined directions are presented in Table S1.

For samples from the non-reddish sites (14XL19, 15XL23, and 15XL24), thermal demagnetization reveals a low-temperature component (LTC) and an intermediate-temperature component (ITC) with laboratory unblocking temperatures between 130°C and ~250-330°C, and between ~290-330°C and 570°C, respectively (Figs. 2a-d, i-p). Most samples from 14XL20 have one component with wide unblocking temperature range from 130°C to 670°C (Figs. 2e, h). Interestingly, one sample from this site reveals a LTC isolated between 130°C and 250°C, and an ITC isolated between 250°C and 530°C (Fig. 2f). Another sample yields three components with a LTC (130-290°C), an ITC (290-530°C) and a high-temperature component (HTC, 530-630°C) (Fig. 2g). The LTCs from these two samples are of normal polarity, whereas the ITCs have reverse polarity (Figs. 2f, g).

Directions of the LTCs from the reddish sites (14XL20) are similar to those of the LTCs from the other sites (14XL19, 15XL23, and 15XL24). Together, they define a mean direction of $D_g = 7.3°$, $I_g = 49.7°$ ($n = 38$, $k = 76.2$, $a_{95} = 2.7°$, “g” stands for geographic coordinates), which is indistinguishable from the direction of the present day magnetic field (PDF) at the sampling localities (Fig. 3a). The ITCs from 14XL19, 15XL23, and 15XL24 yield a mean direction of $D_g = 103.3°$, $I_g = 89.1°$ ($n = 36$, $k = 106.8$, $a_{95} = 2.3°$) before tilt correction, and $D_s = 36.4°$, $I_s = 46.9°$ ($n = 36$, $k = 135.2$, $a_{95} = 2.1°$, “s” stands for stratigraphic coordinates) after 100% tilt correction (Figs. 3a, b). The mean direction of the two ITCs of reverse polarity from 14XL20 is nearly antipodal to the mean direction of the other components isolated from samples in this
site before tilt correction (Fig. 3a). After tilt correction, the mean direction of the isolated components is Ds = 8.2°, Is = 63.6° (n = 15, k = 101, a95 = 3.8°) (Fig. 3b). Robust field tests are not available for these carbonate rocks because the bedding orientation is uniform and the isolated remanence components are mostly of normal polarity. Therefore, we applied the following rock magnetic and petrographic tests to identify potential remagnetization and investigate the origin of the remanence.

186

187 **4. Rock Magnetism**

188 To characterize the magnetic mineralogy and grain size of the limestones, a series of rock
189 magnetic experiments were conducted at the Utah Paleomagnetic Center at the University of
190 Utah (USA), and the Institute for Rock Magnetism at the University of Minnesota (USA). A
191 MicroMag Princeton Measurements vibrating sample magnetometer (VSM, nominal
192 sensitivity of 5 x 10<sup>-9</sup> Am<sup>2</sup>) was used for magnetic hysteresis measurements on 21 samples
193 (nine from 14XL19, three from 14XL20, six from 15XL23, and three from 15XL24) at room
194 temperature and 1 sample (14XL1904) at 29 low temperature steps (20-300 K with temperature
195 increment of 10 K). High-temperature magnetic susceptibility measurements were performed
196 in air on seven whole-rock samples using a KLY-2 KappaBridge AC susceptibility meter with
197 an AC field of 300 A/m and a frequency of 920 Hz. Five cycles of successive heating to 300°C,
198 400°C, 500°C, 600°C, and 700°C with intervening cooling to room temperature were carried
199 out on one sample. Two samples were subjected to the first four cycles up to 600°C, and the
200 remaining three samples underwent successive cycles to 300°C, 500°C, 600°C, and 700°C.
201 Low-temperature remanence and alternating current (AC) susceptibility experiments were run
202 on a Quantum Design Magnetic Properties Measurement System (MPMS-5S). Methods used
203 for the analyses on the MPMS are the same as those described in Huang et al. (2017b). Four
204 samples (one from each of the four sites) were subjected to low-temperature magnetic
205 characterization, beginning with room-temperature saturation isothermal remanent
206 magnetization (RTSIRM given in 2.5 T) measured in zero field during cooling to 20 K and on
207 rewarming back to RT. Next, samples were field cooled (FC) to 20 K in 2.5 T field, the field
208 was removed upon reaching 20 K, and they were measured in zero field on rewarming to RT.
209 Finally, they were zero-field cooled (ZFC) to 20 K, given a low-temperature saturation
210 isothermal remanent magnetization (LTSIRM) in 2.5 T at 20 K, and measured in zero field
211 during warming to RT. Four more samples (two from each of 14XL19 and 15XL23) were
212 characterized using an abbreviated subset of experiments (RTSIRM cooling and ZFC warming
213 measurements), and three samples (one from each of sites 14XL19, 15XL23, and 15XL24)
214 were run for low-temperature susceptibility measurements at five frequencies (1.0, 5.6, 31.6,
215 177.9, and 997.3 Hz).

216

217 **4.1. Room-temperature hysteresis measurements**

218 Hysteresis curves for the specimens from the non-reddish sites 14XL19, 15XL23, and 15XL24
219 are ‘wasp-waisted’ (Figs. 4a-f), indicating the existence of multiple magnetic components with
220 distinct coercivities, which may correspond to mixtures of magnetic minerals, or different size
221 fractions of a single mineral (Tauxe et al., 1996). The low coercive force (Bc) and remanent
222 coercivity (Bcr) values for these specimens indicate that the dominant magnetic carrier has a
223 low coercivity (Table S2). Some of these specimens are saturated at 0.5 T, others show a slow
224 increase of magnetization up to 1 T. These observations suggest that the dominant magnetic
225 carriers in these limestones are probably magnetite with contrasting grain sizes. A small
226 fraction of hematite is also indicated in some specimens. On the Day plot (Fig. 4i), hysteresis
227 parameters of specimens from these sites plot in the region with mixtures of superparamagnetic
228 (SP) and single domain (SD) or pseudosingle domain (PSD) magnetite (Dunlop, 2002). Their
229 Day plot positions thus overlap with the range of data from remagnetized Tethyan Himalayan
230 limestones and remagnetized carbonates elsewhere, and differ from that of the carbonates with
231 primary remanent magnetization (e.g., Jackson and Swanson-Hysell, 2012; Roberts et al., 2013;
232 Huang et al., 2017b). In contrast, specimens from the reddish site 14XL20 are characterized by
233 rectangular hysteresis loops and very high remanent coercivity values (Figs. 4g, h; Table S2),
234 consistent with the dominance of hematite in these specimens.
235

236

237 **4.2. High-temperature susceptibility**

238 High-temperature magnetic susceptibility versus temperature curves of most specimens are
239 characterized by two quasi-linear and reversible descending trends of the susceptibility with an
239 inflection point at ~500°C (Figs. 5b-d). Below 500°C, these specimens lost >50% of their
susceptibility. Above 500°C, the susceptibility decrease shows a steeper slope, with no sharp drop in magnetic susceptibility. This is clearly shown in most specimens, except for 15XL2318, which shows a prominent decrease in magnetic susceptibility near the Curie temperature of magnetite. These observations are consistent with the presence of fine-grained magnetite and minor hematite in specimens from the non-reddish sites 14XL19, 15XL23, and 15XL24, and the dominance of fine-grained hematite in specimens from the reddish site 14XL20 (Dunlop and Ödemir, 1997). Remarkably, three specimens show strikingly different behavior above 400°C (Figs. 5a, e, f). For 15XL2405, a large increase in magnetic susceptibility is observed at 425-570°C on heating curves, followed by a sharp decrease at higher temperatures. The cooling curve is irreversible. This behavior is attributed to magnetite production due to the breakdown of pyrite (Zegers et al., 2003). For 14XL1908 and 15XL2318, a distinct drop in magnetic susceptibility on heating with irreversible cooling paths takes place between 400 and 500°C, suggesting finer grain size magnetite.

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## 4.3. Low-temperature magnetic properties

### 4.3.1. Low-temperature hysteresis measurements

Hysteresis loops of 14XL1904 show strong and progressive rise in remanent saturation ($M_s$), saturation magnetization ($M_s$), $B_c$, and $B_{cr}$ values as the SP fraction of magnetite blocks when the measurement temperature reduces stepwise from room temperature to 20 K (Fig. 6; Table S3). The constriction of the hysteresis loops gradually weakens weaker and loops transform to pot-bellied shapes at low temperature. Upward shift and broadening of the $M_{rh}$ curves with decreasing temperature shows that the hysteresis loops widen (Fig. 6c). Quantitative loop analysis indicates that $M_s$, $M_{rs}$, $B_c$, and $B_{cr}$ values increase to ~2 times their room-temperature values at 40 K (Figs. 6d, e). The sharp increase in $M_s$ value below 40 K is possibly induced by non-linear paramagnetic signal that affects the approach-to-saturation calculations, or ordering of some iron-bearing phases. The shape parameter $\sigma$ shows that the dramatic change from ‘wasp-waisted’ loop shapes to pot-bellied shapes occurs below 30 K (Fig. 6f). These observations are consistent with a gradual transition of the SP-stable single domain (SSD) threshold toward smaller magnetite grain sizes on cooling.

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269
270 **4.3.2. Low-temperature cycling of RTSIRM and SIRM Warming**

271 The low-temperature cooling curves of a RTSIRM for most specimens from the non-reddish
272 sites 14XL19, 15XL23, and 15XL24 show increases in remanence on cooling, followed by a
273 small drop in magnetization at ~225 K, which is also clearly shown in the derivative curves
274 (Figs. 7a-d, i-l; 8a, i), and is thus probably related to the Morin transition in fine-grained
275 hematite. The fact that the drop is lower than the classic Morin transition at 250-260 K likely
276 relates to the finer hematite grain size in these limestones (Özdemir et al., 2008). After this
277 small drop, the remanence of these specimens remains stable or decreases slightly to 175 K,
278 and then descends quickly. However, derivatives of the RTSIRM cooling curves do not show
279 the characteristic magnetite Verwey transition (T<sub>v</sub>) except for specimens from 15XL2304,
280 although magnetite is the dominant magnetic carrier. When the temperature falls below 40 K,
281 a sharp decrease of the remanence appears in some samples (Figs. 7a-b, 8a) and is clearly
282 expressed in the derivative curves (Figs. 7i-k, 8i-k). It is possibly associated with the Besnus
283 transition in monoclinic pyrrhotite at ~34 K (Dekkers et al., 1989). A prominent feature in the
284 warming curves of the RTSIRM is a “hump” in remanence between 150 K and 200 K, which
285 is the hallmark feature of oxidized magnetite (Özdemir and Dunlop, 2010). Specimens
286 15XL2312 and 15XL2317 (Figs. 8b-c) show a monotonic increase in remanence until 40 K in
287 the cooling curves and reversible warming curves, suggesting strong maghemitization of
288 magnetite, and existence of goethite (Dunlop and Ödemir, 1997). Derivatives of the RTSIRM
289 cooling curves of 15XL2312 have a peak at 110-120 K, indicating a small contribution of
290 magnetite. The low-temperature cycling curves of a RTSIRM for specimen from the reddish
291 site 14XL20 undergo a progressive decrease in remanence on cooling, with a small amount
292 (~6%) of loss in remanence when warming back to room temperature and no Morin transition
293 displayed (Fig. 8d, l), suggesting that the dominant hematite grain is nanosized (De Boer et al.,
294 2001).

295

296 The low-temperature SIRM warming curves after ZFC and FC treatment for specimens from
297 the non-reddish sites 14XL19, 15XL23, and 15XL24 show a continuous decay in remanence
during warming to room temperature with more than half of the low-temperature remanence at
298 20 K erased by warming to room temperature (Figs. 7e-h, 8e-g), implying that more than half
299
of the ferromagnetic material is superparamagnetic at room temperature and does not contribute to the NRM. FC remanence is stronger than that of the ZFC, but the difference is very small for most specimens from sites 14XL19, 15XL23, and 15XL24 (Figs. 7g-h). No clear Morin transition is recognized, but some specimens show a very weak Verwey transition as distinguished in the derivatives of the FC curves (Figs. 7j, k; 8i-k). Large separation between FC and ZFC curves can be seen in 15XL2312, and possibly also in sample 15XL2317, which is typically attributed to the strong contribution of goethite. 14XL2018 from the reddish site shows characteristic concave FC and ZFC curves, these curves are also separated (Fig. 8h), indicating small contribution of goethite besides dominant fine-grained hematite. For ZFC curves showing no Verwey transition, magnetite grain size distribution can be calculated using the method of Worm and Jackson (1999). The inferred broad, unimodal size distribution of the SP magnetite (Figs. 7m, n), peaking at ~$10^{-24}$ m³ (10 nm in diameter), is in good agreement with the modelled SP-SD or SP-PSD mixing curves in Fig. 4i (Dunlop, 2002).

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### 4.3.3. Low-temperature AC susceptibility

The ferrimagnetic in-phase susceptibility ($\chi'$) at multiple frequencies for all specimens from the non-reddish sites 14XL19, 15XL23, and 15XL24 is presented after subtraction of the paramagnetic contributions based on Curie-Weiss paramagnetic temperature dependence (Fig. 9). At room temperature, a relatively strong frequency dependence of susceptibility is clearly shown with $\chi'$ at 1.0 Hz >20% higher than that at 997.3 Hz, indicating a significant fraction of grains with diameters below 15-25 nm (Worm, 1998). $\chi'$ for all specimens shows sharper increase on warming below 40 K (especially for 14XL1904), which may be attributed to the presence of minor pyrrhotite, as also shown by RTSIRM cooling curves (Figs. 7, 8). After that, a nearly linear increase in $\chi'$ at higher temperatures is observed. Together with the strong frequency dependence of $\chi'$ over the entire temperature range, a wide distribution of grain sizes/unblocking temperatures for magnetite is implied. Out-of-phase susceptibility ($\chi''$) is roughly 10 times lower than $\chi'$, and is noisier with increasing measurement frequency (Fig. 9). A sharp peak in $\chi''$ observed below 40 K for all frequencies may also be induced by pyrrhotite. At higher temperatures, $\chi''$ mimics the linear increase of $\chi'$ with the magnitude doubled from 50 K to 300 K. In general, $\chi''$ at each temperature is in excellent quantitative agreement with
the calculated values of $-(\pi/2) d\chi'/d(\ln(f))$ from Néel's single-domain theory (f is the measurement frequency), implying that $\chi''$ originates from short-period viscosity of SP particles.

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In summary, rock magnetic analyses suggest that the dominant magnetic carrier for the non-reddish sites (14XL19, 15XL23, and 15XL24) is magnetite with broad grain sizes and blocking temperatures. Over half of these grains are superparamagnetic at room temperature, inducing 'wasp-waisted' hysteresis loops, blurred to suppressed Verwey transition, and strong frequency-dependent magnetic susceptibility. Maghemitization is likely widespread in these grains. Fine-grained hematite is apparently common in these limestones. Small amounts of pyrrhotite and goethite are also apparent in some specimens. Pyrite, which alters on heating, is also indicated by the behavior of some specimens. The reddish site 14XL20 most likely contains fined-grained hematite, with a suppressed Morin transition, as the dominant magnetic carrier and goethite as a minor magnetic phase.

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## 5. SEM observation and EDS analysis

To visually and chemically characterize the magnetic minerals and their morphologies, which may provide an essential clue for understanding their parageneses, we examined eight polished thin sections of specimens from these sites (three from 14XL19, one from 14XL20, three from 15XL23, and one from 15XL24) under the SEM at Peking University (Beijing, China) as used in a previous study (Huang et al., 2017a). EDS analysis was subsequently conducted after backscattered electron images were obtained.

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For specimens from the non-reddish sites 14XL19, 15XL23, and 15XL24, the principal magnetic mineral identified under SEM is magnetite (Fig. 10; Fig. S1). The magnetite grains are subeuhedral to euhedral or framboidal in shape with sizes less than 15 µm. Some magnetite shows growth zoning (Fig. 10k) and as rims surrounding pyrite grains (Figs. 10j). Pyrite is present as independent euhedral or framboidal grains (Fig. 10a, n), or is distributed typically in cores of fine-grained magnetite crystals in the framboids (Figs. 10b-f, i-l). These phenomena suggest that the magnetite grains are pseudomorphic after pyrite, and are probably an oxidation product of pyrite during later diagenetic events. Subrounded to rounded homogeneous
magnetite is rarely found in the limestones. Minor rutile grains contain little iron (Fig. 10m), which may indicate that they formed after iron was leached from the lattice of detrital titanomagnetite during dissolution and diagenesis. The Fe/S ratios of some iron sulfide grains with diameter of ~1 µm are between 0.5 and 1, suggesting a chemical composition of pyrrhotite or a mixture of pyrrhotite and pyrite (Figs. 10g, h). These Fe-S grains were possibly formed during desulphidation of pyrite (e.g., Rochette, 1987). Minor hematite or goethite cannot be discriminated from magnetite by SEM observations and EDS analyses in these specimens. For specimen from the reddish site 14XL20, euhedral or framboidal aggregates composed of microcrystalline hematite and minor goethite are recognized by their characteristic platy morphology (Figs. 10o-t). The inner part of some aggregates are pseudomorphic after authigenic magnetite (Figs. 10p, t). These observations indicate that hematite or goethite are the additional oxidation products of pyrite.

## 6. Discussion

### 6.1 Evidence for the Remagnetizations

Small variations in bedding orientation and the absence of a remanence at dual polarity of the sampled sites make it impossible to estimate the age and origin of the NRM by classic field tests. However, our comprehensive rock magnetic and petrographic results unequivocally indicate that these limestones were remagnetized due to growth of authigenic magnetic minerals and destruction of any pro-existing magnetic phases, which is a very prevailing mechanism for remagnetization (e.g., Elmore et al., 2012).

The dominant magnetization carrier of the non-reddish sites 14XL19, 15XL23, and 15XL24 is magnetite with a broad range of unblocking temperature (Fig. 5). A large fraction of these magnetite grains is superparamagnetic as indicated by ‘wasp-waisted’ hysteresis loops at room temperature, by the loss of most of LTSIRM when warming up to room temperature, by a suppressed Verwey transition, and by strong frequency-dependence of magnetic susceptibility (Figs. 4-9). These features commonly fingerprint remagnetized carbonate rocks (Jackson and Swanson-Hysell, 2012, and references therein). Although the SP fraction does not contribute to NRM at room temperature, it is considered to have the same origin as the SSD or PSD particles which do carry the remanence in remagnetized carbonate rocks. Significant SP
contribution and unimodal size distribution peaking at SP-SSD threshold below room
temperature have been sometimes observed in carbonate rocks with extraterrestrial or
extracellular magnetite (e.g., Lanci and Kent, 2006), but they are more often attributed to
authigenesis in carbonate rocks (Jackson and Swanson-Hysell, 2012). Our SEM observations
fully support that alteration of pre-existing pyrite and neoformation of magnetite are the reasons
for remagnetization in the limestones we have examined (Fig. 10). More than half of the
authigenic magnetite grains never grew larger than 25 nm in size, and are responsible for ‘wasp-
waisted’ hysteresis loops and frequency-dependent susceptibility. The rest likely grew through
the SP-SSD threshold at room temperature, inducing acquisition of a stable chemical remanent
magnetization (CRM). The Verwey transition is suppressed for authigenic magnetite, which is
possibly related to its nonstoichiometric character due to maghemitization and its origin from
pyrite oxidization (Jackson and Swanson-Hysell, 2012). The appearance of a weak Verwey
transition in the RTSIRM cycling and LTSIRM warming curves of some specimens (Figs. 7,
8), especially these from site 15XL23, however, possibly indicates the existence of a small
amount of detrital/biogenic magnetite, which may carry a primary NRM. The rutile grains we
observe under SEM contain some iron, suggesting they might have formed by leaching of iron
from detrital titanomagnetite during dissolution and alteration (Fig. 10). However, this
potentially primary remanence overlaps with the unblocking temperatures and coercivities of
the predominant authigenic magnetite, which probably makes it impossible to isolate it by
thermal or alternating field demagnetization.

411

The reddish site 14XL20 is also remagnetized, as its main magnetic carriers are authigenic
hematite and minor goethite pseudomorphic after pyrite. No detrital hematite was identified in
these rocks. The remanent magnetization directions of this site are very different from these of
other sites in both geographic and stratigraphic coordinates (Fig. 3), likely indicating another
episode of remagnetization. Secondary magnetizations carried by hematite have been reported
in a few studies (e.g., Elmore et al., 1993; Torsvik et al., 2005), and the rock magnetic character
of these rocks and origin of hematite are not clear. Here we show that most hematite grains are
nanoscaled with wide a unblocking temperature range (Figs. 5, 8). These hematite aggregates
retain the morphology of pyrite, similar to authigenic magnetite pseudomorph from pyrite,
suggesting they result from further oxidation products of pyrite under higher oxygen fugacity.
Two specimens from this site yield an ITC of reverse polarity (Fig. 3a). A robust reversals test
cannot be applied because the number of independent results is too small, but it is quite clear
that these two ITCs are nearly antipodal to the ITCs/HTCs of other specimens in this site. This
suggests that the growth of authigenic hematite spanned a sufficiently long time in some
specimens during both polarities. The classic reversals test for these sites can therefore appear
to be positive although both normal and reversed polarities are carried by remagnetizations.

The directions of the LTC isolated below 330°C from these limestones differ from the ITC and
HTC, but are indistinguishable from the PDF (Fig. 3a). This remanence is probably dominated
by a recent thermoviscous component carried by authigenic magnetite or hematite. According
to Néel's theory, this recent viscous overprint should be completely removed by 200°C. The
observed anomalously high unblocking temperatures may be attributed to the PSD/MD
behavior of some authigenic magnetite and maghemitization (Jackson and Worm, 2001).
Additionally, monoclinic pyrrhotite, recognized by our RTSIRM cycling curves (Figs. 7, 8) and
SEM & EDS examinations (Fig. 10), may make a small contribution to the LTC. Production of
authigenic monoclinic pyrrhotite is usually linked to diagenesis and metamorphism of pyrite-
bearing sediments (Roberts, 2015), such as those found in metamorphosed belts of the
Himalaya (Appel et al., 2012). This process is, however, extremely slow below 250°C. The
Triassic limestones in the Yushu-Nangqian area show no sign of metamorphism. Thus, it is
likely that it occurred over the hundreds of millions of years since the deposition of these
Triassic limestones.

## 6.2. Mechanism and Timing of the Remagnetizations

Based on our paleomagnetic, rock magnetic, and petrographic experiments, we distinguish
three episodes of remagnetization in the Upper Triassic limestones in the Yushu-Nangqian area.
The first one is expressed by oxidation of pyrite and neoformation of magnetite. We argue that
it may be related to the collision between Qiangtang block and northeastern Tibet after the
closure of Paleo-Tethys Oceans in the Late Triassic (Yin and Harrison, 2000; Song et al., 2015).
Indeed, the ITCs isolated from the non-reddish sites 14XL19, 15XL23, and 15XL24 give a
mean direction of Ds/Is = 36.4°/46.9°. Comparing this direction to the robust Late Triassic paleopole of the Qiangtang block determined from volcanic rocks (204–213 Ma, Song et al., 2015) indicates a bedding dip of ~7° at that time, suggesting that these limestones was only slightly tilted during the remagnetization. This is consistent with an early tectonic event initially bringing the limestones above sea level which would have dramatically changed the redox condition from reductive to oxidizing, inducing pyrite oxidation. As proposed for the Appalachian orogenic belt, this process may have been facilitated by orogenic fluids, either pumped by tectonic force or gravity; such fluids may have circulated through significant parts of the crust in the orogenic belt.

The secondary episode of remagnetization produced authigenic hematite and minor goethite in the reddish site. The distribution of these reddish limestones just below the unconformity between the Cenozoic red reds and Triassic strata strongly suggests that this episode of remagnetization was related to the development of a localized intra-mountain basin in northeastern Tibetan Plateau soon after India-Asia collision at ~58 Ma (e.g., Hu et al., 2015). Considering a quasi-stable, coupled position of the Lhasa and Qiangtang blocks at ~24°N at the sampling locality since early Cretaceous times (Lippert et al., 2014), the mean direction of this remanence of Dg/IG = 292°/53.6° implies that remagnetization occurred when bedding was tilted to ~30°. Compared to the expected declination at the sampling locality from the Eurasian APWP (Torsvik et al., 2012), this further implies a counterclockwise rotation of ~75° occurring after the remagnetization. The third episode is dominated by a recent thermoviscous overprint carried by authigenic magnetite or hematite initially formed in the first and secondary remagnetization episodes, and a minor CRM carried by authigenic monoclinic pyrrhotite with a protracted formation duration.

## 6.3 Implications for the preservation of primary NRM in the Tethyan carbonate rocks in the Tibetan Plateau

Preservation of primary NRM in sedimentary rocks depends on the long-term fate of magnetic minerals in the sediment during diagenesis. To discuss whether the Tethyan carbonate rocks in Tibet retain a primary NRM, we review those carbonate rocks that typically retain a primary NRM and provide high-fidelity records of the geomagnetic field (e.g., Channell and McCabe,
1994; Abrajevitch and Kodama, 2009; Chang et al., 2018). Those rocks are predominantly deep-water pelagic marine carbonates deposited far from the continents at water depths of 3000-6000 m and with depositional rates of <10 cm/kyr. Magnetic carriers in these rocks are dominated by biogenic SD magnetite and/or detrital PSD-MD magnetite and minor hematite (both aqueous and aeolian delivery). The early diagenesis in pelagic environments is more likely to be oxic because the organic matter is of low concentration (<0.2%) in these areas with low primary productivity and sedimentation rate and it can be consumed by the dissolved oxygen from the overlying bottom water (Roberts et al., 2013). Dissolved iron in pelagic bottom waters is limited (Boyd and Ellwood, 2010), so the formation of authigenic hematite or goethite is unlikely. Therefore, little diagenetic alteration affect the biogenetic or detrital magnetite or hematite, and the primary NRM is not likely to be contaminated by the CRM acquired by authigenic magnetic minerals. In the oxic conditions of later diagenesis or subaerial weathering, authigenic magnetic minerals, such as hematite or goethite, may still form at the expense of biogenetic or detrital magnetite in pelagic carbonates, leading to alteration of the primary NRM.

The Tethyan carbonates in the Tibetan Plateau were mostly deposited in shallow-water carbonate platform, ramp, and shelf settings with depths less than 350 m and depositional rate of ~100 cm/kyr (Yin and Harrison, 2000). Organic carbon contents in these sediments could be as high as 10%, considering the high productivity at low latitudes where they were mostly located since the Carboniferous (Fig. 1b). Post-depositional degradation of the organic matter by microbial respiration consumes oxygen (oxic diagenesis), nitrate, manganese oxides, iron (oxyhydr)oxides, and sulphate in sequence until either all oxidants or all reactive organic matter is used up (Roberts et al., 2013). Therefore, in such environments, a sulphidic diagenetic environment would be created, during which detrital magnetite or hematite would be dissolved and iron sulphides, such as paramagnetic pyrite and minor ferrimagnetic greigite, would be formed by reaction of dissolved Fe²⁺ with dissolved HS⁻. The primary NRM could thus be destroyed or overprinted during the early diagenesis. This process is ubiquitous in shallow-water carbonates with high organic carbon fluxes, and it explains why detrital (titano)magnetite and hematite are rarely preserved in the Triassic limestones we studied as well as Jurassic-
512 Paleogene Tethyan Himalayan limestones (Huang et al., 2017a,b), although the detrital input
513 is higher in the shallow-water carbonates than that in the pelagic carbonates. In the later
514 diagenetic stage, when the Tethyan carbonate rocks were tectonically uplifted above sea level
515 due to orogenesis, following closure of Tethys Oceans, diagenesis changed from anoxic to
516 suboxic and oxic, leading to oxidation of pyrite/greigite to magnetite/hematite/goethite and
517 acquisition of CRM as identified in this study and the Tethyan Himalayan limestones (Huang
518 et al., 2015, 2017a,b). This process was probably assisted by continental-scale circulation of
519 orogenic fluids through the shallow-water carbonates with high porosity (Jackson and
520 Swanson-Hysell, 2012). Formation of authigenic monoclinic pyrrhotite during metamorphism
521 is another important mechanism for the remagnetization of the metamorphic Tethyan carbonate
522 rocks (Appel et al., 2012). Other mechanisms involving clay diagenesis in burial (e.g., Katz et
523 al., 2000), organic maturation (Blumstein et al., 2004), and pressure solution (e.g., Elmore et
524 al., 2006) may also have occurred in the Tethyan carbonates. We speculate that these diagenetic
525 processes are closely linked to the crustal shortening in the Tibetan Plateau and may have acted
526 in concert with previous remagnetization processes, producing magnetic mineral alteration, and
527 inducing pervasive remagnetization of Tethyan carbonates.
528

529 There are, however, examples of shallow-water carbonate rocks that retain a primary
530 remanence. One is the Pleistocene Tahiti reef with significant detrital magnetite input from
531 basalt of the nearby island (Ménabréaz et al., 2010). The other is Jurassic carbonate rocks in
532 the Paris basin, which contain a significant amount of biogenic and detrital magnetite, as well
533 as authigenic magnetite and goethite. There a refined paleomagnetic demagnetization
534 procedure of AF (<10 mT), followed by thermal (up to 100-150°C), and then AF
535 demagnetization enabled the isolation of a primary remanence (Belkaaloul and Aïssaoui, 1997).
536 In both examples, biogenic and/or detrital magnetite are found in exceptional abundance in
537 these rocks. In most cases unfortunately, the contribution of the authigenic magnetite to the
538 NRM cannot be distinguished from that of the primary biogenic and detrital (titano)magnetite,
539 and a primary remanence cannot be sufficiently isolated. Considering the identified pervasive
540 remagnetization in Tethyan carbonates in the Tibetan Plateau and contrasting depositional and
541 diagenetic conditions of the Tethyan carbonate rocks compared to the pelagic carbonates,
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detailed rock magnetic and petrographic studies are strongly advised to identify the primary or
secondary nature of the NRM, in addition to classic fold or reversals tests that may not facility
to the detection of remagnetized rocks.

7. Conclusions

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Paleomagnetic data for of carbonate rocks have played an important role in paleogeographic reconstructions of the Tibetan Plateau, yet these rocks often yield controversial results. We applied comprehensive rock magnetic measurements and SEM & EDS analyses to samples from upper Triassic carbonate rocks exposed in the Yushu-Nangqian area of the eastern Qiangtang block. Detrital magnetite, which may have carried the primary NRM, is rarely preserved in these rocks. On the contrary, we identified a main episode of regional remagnetization residing in authigenic magnetite (pseudomorphic after pyrite) from three non-reddish sites (14XL19, 15XL23, 15XL24), characterized by ‘wasp-waisted’ hysteresis loops, suppressed Verwey transitions, and strong frequency-dependent magnetic susceptibility. We propose that this remagnetization phase occurred early, in the Late Triassic, during the collision between the Qiangtang block and northeastern Tibetan Plateau. A second episode of remagnetization is recognized in one reddish site (14XL20) residing in authigenic hematite and minor goethite pseudomorphic after pyrite as well. This remagnetization is interpreted to relate to the localized Cenozoic basin development soon after India-Asia collision. The third remagnetization is a combination of a recent thermoviscous overprint carried by authigenic magnetite and a CRM carried by authigenic pyrrhotite that may have formed very slowly since the Triassic. These episodes of remagnetization, if not recognized, would mistakenly yield either a lower paleolatitude (from the non-reddish sites 14XL19, 15XL23, and 15XL24) or higher paleolatitude (from the reddish site 14XL20) of the Qiangtang block after 100% bedding correction. Comparison between shallow-water Tethyan carbonate rocks in the Tibetan Plateau and primary remanence-carrying pelagic carbonate rocks indicates that they are very different in depositional and diagenetic conditions. Diagenesis has had limited effects on these pelagic carbonate rocks, whereas it had a controlling influence on the shallow-water carbonates on the continental margin. The possibility of preservation of primary NRM in the Tethyan carbonates is thus not promising, but rock magnetic tests and petrographic examinations are necessary to
572 diagnose insidious remagnetization.

573

574 Acknowledgments

575 Data to support this article are available in supplementary material Tables S1–S3. This project
576 was funded by the Netherlands Organization for Scientific Research (NWO) with a Rubicon
577 grant (825.15.016) to W.H. The first author was also supported by a visiting research
578 fellowship from the Institute for Rock Magnetism (IRM) at the University of Minnesota, which
579 is funded by the Instruments and Facilities program of NSF. G.D.-N. acknowledges support
580 from ERC consolidator grant MAGIC 649081 and the project of “Probing Deep and Surface
581 Processes in Central Tibet” from Agence National de la Recherche, ANR, France. We thank
582 Peter C. Lippert at Utah Pmag Center and Peat A. Solheid at the IRM for laboratory assistance.
583 Pierrick Roperch is especially appreciated for collecting the samples, applying thermal
demagnetization, and sharing his ideas about the remanence acquisition in carbonates.
584
585 Comments from Prof. John W. Geissman have greatly improved this manuscript.

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601 **Figure captions**
Figure 1. (a) Tectonic map of the India-Asia collision zone (modified from van Hinsbergen et al. (2012)). (b) Paleolatitude versus time plot with paleomagnetic data from the Tibetan Himalaya, Lhasa, and Qiangtang blocks in the Tibetan Plateau generated using www.paleomagnetism.org (Koymans et al., 2016). Circles represent paleomagnetic poles determined by carbonate rocks. Dots are from clastic and volcanic rocks. Paleomagnetic data are from Song et al. (2017), updated with the latest reported poles (Bian et al., 2017; Ma et al., 2018, 2019; Cao et al., 2019). (c) Geologic map of the Yushu-Nangqian area with paleomagnetic sampling localities of the Upper Triassic limestones plotted.

610

Figure 2. Demagnetization diagrams for representative specimens. All diagrams are displayed after bedding tilt correction. Closed (open) symbols represent the projection of vector endpoints on the horizontal (vertical) plane; values represent the thermal demagnetization step. Lines are fitted directions for the isolated remanence. Blue lines are low-temperature components. Green lines are intermediate-temperature from sites 14XL19, 15XL23, and 15XL24; red lines are intermediate-temperature and high-temperature components from site 14XL20.

617

Figure 3. Equal-area projections of the isolated remanent magnetization directions by line fits in geographic (a) and stratigraphic coordinates (b). Blue dots are low-temperature components from all sites; green dots are intermediate-temperature component from sites 14XL19, 15XL23, and 15XL24; red dots (circles) are isolated remanence from site 14XL20. Small black dots (circles) represent the mean remanence directions, big circles are the corresponding 95% confidence limit. The two remanence directions from site 14XL20 with reversal polarity in geographic coordinates were transferred to normal polarity for the calculation of the mean direction in stratigraphic coordinates.

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Figure 4. (a-h) Room-temperature magnetic hysteresis loops from representative specimens after correction for the paramagnetic/diamagnetic/antiferromagnetic contribution. All the loops for specimens from sites 14XL19, 15XL23, and 15XL24 are 'wasp-waisted'. (i) Day plot of the hysteresis parameters for specimens from sites 14XL19, 15XL23, and 15XL24 in this study (blue dots), and the remagnetized Jurassic-Paleogene Tethyan Himalayan limestones (triangles
and diamonds) presented in Huang et al. (2017b).

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Figure 5. Multicycle high-temperature bulk magnetic susceptibility measurements in air for representative specimens.

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Figure 6. (a-c) Low-temperature magnetic hysteresis loops (a), back field curves (b), and $M_{rh}$ curves (c) for specimen 14XL1904 measured at 29 steps. Hysteresis loops have been corrected for paramagnetic contribution. (d-f) Saturation magnetization ($M_s$), saturation remanent magnetization ($M_{rs}$), coercive force ($B_c$), remanent coercive ($B_{cr}$), and shape parameter ($\sigma$) as a function of decreasing temperature for 14XL1904.

642

Figure 7. (a–d) Low-temperature RTSIRM cycling curves at zero field for representative limestone specimens from sites 14XL19 and 15XL24. (e–h) Corresponding low-temperature RTSIRM warming curves in zero field after ZFC and FC treatments. (i–l) Corresponding derivatives of the RTSIRM cooling and ZFC SIRM warming curves. (m, n) Particle size distribution calculated from the ZFC SIRM curves in “e” and “h” (Worm and Jackson, 1999).

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Figure 8. (a–d) Low-temperature RTSIRM cycling curves at zero field for representative limestone specimens from sites 15XL23 and 14XL20. (e–h) Low-temperature SIRM warming curves in zero field after ZFC and FC treatments for specimens in “a”-“d”. (i–l) Corresponding derivatives of the RTSIRM cooling and ZFC SIRM warming curves.

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Figure 9. (a, c, and e) Low-temperature in-phase and quadrature AC susceptibility measured from 20 to 300 K at different frequencies (1.0, 5.6, 31.6, 177.9, and 997.3 Hz) for 3 representative limestone specimens from three sites. (b, d, and f) Expanded view of quadrature susceptibilities, compared to the derivative of in-phase susceptibility with respect to ln(f).

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Figure 10. SEM backscattered electron images of specimens from the Upper Triassic limestones. Mag: magnetite, Hem: hematite, Po: pyrrhotite, Cal: calcite, Py: pyrite, and Rt: rutile. White dots are the EDS analysis spots with the results shown in Figure S1.

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<figure><img src="image_3.png" /><figcaption>Figure 1</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Figure 2</figcaption></figure>
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<figure><img src="image_42.png" /></figure>

RT remanence on warming

RT remanence on cooling

FC, remanence

ZFC, remanence

Derivative of ZFC, remanence

Derivative of RT remanence on cooling
<figure><img src="image_43.png" /></figure>

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<figure><img src="image_62.png" /><figcaption>Figure 10</figcaption></figure>